There was some code replication around getting string and integer
values out of an attribute map.  This adds new methods to the
TAttributeMap class to encapsulate some accessor details.

Fixes #1091.

Only track variables in the global scope

The semantics differ between GLSL/HLSL and SPIR-V.
Translate between these.

HLSL: split textures used for both shadow and non-shadow modes

A single texture can statically appear in code mixed with a shadow sampler
and a non-shadow sampler.  This would be create invalid SPIR-V, unless
one of them is provably dead.

The previous detection of this happened before DCE, so some shaders would
trigger the error even though they wouldn't after DCE.  To handle that
case, this PR splits the texture into two: one with each mode.  It sets
"needsLegalization" (if that happens for any texture) to warn that this shader
will need post-compilation legalization.

If the texture is only used with one of the two modes, behavior is as it
was before.

HLSL: Fix #954: Track/access subsets of flattened multi-level aggrega…

Works in conjuction with d1be7545 to represent and modify a partially
dereferenced multi-level flattened aggregate.

HLSL: Fix crash when flattening both side of assignement simultaneously.

HLSL: add subpass input types and methods

Add support for Subpass Input proposal of issue #1069.

Subpass input types are given as:

    layout(input_attachment_index = 1) SubpassInput<float4> subpass_f;
    layout(input_attachment_index = 2) SubpassInput<int4>   subpass_i;
    layout(input_attachment_index = 3) SubpassInput<uint4>  subpass_u;

    layout(input_attachment_index = 1) SubpassInputMS<float4> subpass_ms_f;
    layout(input_attachment_index = 2) SubpassInputMS<int4>   subpass_ms_i;
    layout(input_attachment_index = 3) SubpassInputMS<uint4>  subpass_ms_u;

The input attachment may also be specified using attribute syntax:

    [[vk::input_attachment_index(7)]] SubpassInput subpass_2;

The template type may be a shorter-than-vec4 vector, but currently user
structs are not supported.  (An unimplemented error will be issued).

The load operations are methods on objects of the above type:

    float4 result = subpass_f.SubpassLoad();
    int4   result = subpass_i.SubpassLoad();
    uint4  result = subpass_u.SubpassLoad();

    float4 result = subpass_ms_f.SubpassLoad(samp);
    int4   result = subpass_ms_i.SubpassLoad(samp);
    uint4  result = subpass_ms_u.SubpassLoad(samp);

Additionally, the AST printer could not print EOpSubpass* nodes.  Now it can.

Fixes #1069

- support C++11 style brackets [[...]]
- support namespaces [[vk::...]]
- support these on parameter declarations in functions
- support location, binding/set, input attachments

Remove opaque workarounds

Also, remove assumption that if something is opaque that it
must be in the UniformConstant storage class.

This allows function declarations to know all parameters will
be in the Function storage class.

This assumes it will be combined with optimizing transforms
that eliminate assignments to opaques.

HLSL: force textures to shadow modes from combined samplers

Texture shadow mode must match the state of the sampler they are
combined with.  This change does that, both for the AST and the
symbol table.  Note that the texture cannot easily be *created*
the right way, because this may not be known at that time.  Instead,
the texture is subsequently patched.

This cannot work if a single texture is used with both a shadow and
non-shadow sampler, so that case is detected and generates an error.
This is permitted by the HLSL language, however.  See #1073 discussion.

Fixed one test source that was using a texture with both shadow and
non-shadow samplers.

Legalization: Fix warnings and disable tests when spirv-tools not present

Enable HLSL legalization

Also added known-good mechanism to fetch latest validated spirv-tools.
Also added -Od and -Os to disable optimizer and optimize for size.

Fetching spirv-tools is optional for both glsl and hlsl. Legalization
of hlsl is done by default if spirv-opt is present at cmake time.
Optimization for glsl is currently done through the option -Os.

Legalization testing is currently only done on four existing shaders.
A separate baseLegalResults directory holds those results. All previous
testing is done with the optimizer disabled.

Implement extension GL_NV_shader_atomic_int64

Travis: use make instead of ninja