HLSL: add methods to handle user structures in texture template type.

Some languages allow a restricted set of user structure types returned from texture sampling
operations.  Restrictions include the total vector size of all components may not exceed 4,
and the basic types of all members must be identical.

This adds underpinnings for that ability.  Because storing a whole TType or even a simple
TTypeList in the TSampler would be expensive, the structure definition is held in a
table outside the TType.  The TSampler contains a small bitfield index, currently 4 bits
to support up to 15 separate texture template structure types, but that can be adjusted
up or down.  Vector returns are handled as before.

There are abstraction methods accepting and returning a TType (such as may have been parsed
from a grammar).  The new methods will accept a texture template type and set the
sampler to the structure if possible, checking a range of error conditions such as whether
the total structure vector components exceed 4, or whether their basic types differe, or
whether the struct contains non-vector-or-scalar members.  Another query returns the
appropriate TType for the sampler.

High level summary of design:

In the TSampler, this holds an index into the texture structure return type table:

    unsigned int structReturnIndex : structReturnIndexBits;

These are the methods to set or get the return type from the TSampler.  They work for vector or structure returns, and potentially could be expanded to handle other things (small arrays?) if ever needed.

    bool setTextureReturnType(TSampler& sampler, const TType& retType, const TSourceLoc& loc);
    void getTextureReturnType(const TSampler& sampler, const TType& retType, const TSourceLoc& loc) const;

The ``convertReturn`` lambda in ``HlslParseContext::decomposeSampleMethods`` is greatly expanded to know how to copy a vec4 sample return to whatever the structure type should be.  This is a little awkward since it involves introducing a comma expression to return the proper aggregate value after a set of memberwise copies.

This generated code needs an optimization pass to eliminate the assignments
to the opaque members.

HLSL: Flatten structs for all non-arrayed I/O interfaces.

Code was reflecting an old historical use of sampled as a SPIR-V
2-valued operand, instead of its current 3 values.

This prevents potentional sharing from inadvertently affecting other arrays.

This will help in expanding flattening and reducing splitting.

HLSL: implement #pragma pack_matrix(layout)

This adds support for #pragma pack_matrix() to the HLSL front end.

The pragma sets the default matrix layout for subsequent unqualified matrices
in structs or buffers. Explicit qualification overrides the pragma value. Matrix
layout is not permitted at the structure level in HLSL, so only leaves which are
matrix types can be so qualified.

Note that due to the semantic (not layout) difference in first matrix indirections
between HLSL and SPIR-V, the sense of row and column major are flipped.  That's
independent of this PR: just a factor to note.  A column_major qualifier appears
as a RowMajor member decoration in SPIR-V modules, and vice versa.

Includes related trackLinkage() code removal and name improvements.

HLSL: fix qualifier propagation from user struct types to block defin…

The HLSL FE tracks four versions of a declared type to avoid losing information, since it
is not (at type-decl time) known how the type will be used downstream.  If such a type
was used in a cbuffer declaration, the cbuffer type's members should have been using
the uniform form of the original user structure type, but were not.

This would manifest as matrix qualifiers (and other things, such as pack offsets) on user struct
members going missing in the SPIR-V module if the struct type was a member of a cbuffer, like so:

    struct MyBuffer
    {
        row_major float4x4 mat1;
        column_major float4x4 mat2;
    };

    cbuffer Example
    {
        MyBuffer g_MyBuffer;
    };

Fixes: #789

This allows removal of isPerVertexBuiltIn(). It also leads to
removal of addInterstageIoToLinkage(), which is no longer needed.

Includes related name improvements.

The goal is to flatten all I/O, but there are multiple categories and
steps to complete, likely including a final unification of splitting
and flattening.

GLSL: Add GLSL 4.60 features

Most of this was obsoleted by entry-point wrapping.
Some other is just unnecessary.
Also, includes some spelling/name improvements.

This is to help lay ground work for flattening user I/O.

HLSL: Non-functional: warning fix + remove unused member

Two non-functional changes:

1. Remove flattenLevel, which is unneeded since at or around d1be7545.

2. Fix build warining about unused variable in executeInitializer.

SPV: Memory qualifiers should decorate top-level block members

HLSL: handle multiple clip/cull semantic IDs

HLSL allows several variables to be declared.  There are packing rules involved:
e.g, a float3 and a float1 can be packed into a single array[4], while for a
float3 and another float3, the second one will skip the third array entry to
avoid straddling

This is implements that ability.  Because there can be multiple variables involved,
and the final output array will often be a different type altogether (to fuse
the values into a single destination), a new variable is synthesized, unlike the prior
clip/cull support which used the declared variable.  The new variable name is
taken from one of the declared ones, so the old tests are unchanged.

Several new tests are added to test various packing scenarios.

Only two semantic IDs are supported: 0, and 1, per HLSL rules.  This is
encapsulated in

     static const int maxClipCullRegs = 2;

and the algorithm (probably :) ) generalizes to larger values, although there
are a few issues around how HLSL would pack (e.g, would 4 scalars be packed into
a single HLSL float4 out reg?  Probably, and this algorithm assumes so).