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).

Fix dref explicit LOD form of sample with cube texture arrays

The dref parameter was being used as the LOD.  Now it it's properly the dref.

HLSL: Fix crash with --resource-set-binding [n] (global form, not per-register form)

--resource-set-binding has a mode which allows per-register assignments of
bindings and descriptor sets on the command line, and another accepting a
single descriptor set value to assign to all variables.

The former worked, but the latter would crash when assigning the values.
This fixes it, and makes the former case a bit more robust against premature
termination of the pre-register values, which must come in (regname,set,binding)
triples.

This also allows the form "--resource-set-binding stage setnum", which was
mentioned in the usage message, but did not parse.

The operation of the per-register form of this option is unchanged.

SPV: Decorate parameters

HLSL: Fix #980: flatten opaque initializers to use aliases.

Fix NULL pointer dereference in TParseContext::builtInOpCheck

Address part B of #982: $Global will use std140 instead of HLSL offsets.