Add D3DCOLORtoUBYTE4 decomposition

HLSL: default function parameters

Also correctly test and handle missing newline.

This PR adds support for default function parameters in the following cases:

1. Simple constants, such as void fn(int x, float myparam = 3)
2. Expressions that can be const folded, such a ... myparam = sin(some_const)
3. Initializer lists that can be const folded, such as ... float2 myparam = {1,2}

New tests are added: hlsl.params.default.frag and hlsl.params.default.err.frag
(for testing error situations, such as ambiguity or non-const-foldable).

In order to avoid sampler method ambiguity, the hlsl better() lambda now
considers sampler matches.  Previously, all sampler types looked identical
since only the basic type of EbtSampler was considered.

Modify shader ballot extension by adding OpSubgroupReadInvocationKHR

Add support for SPV_NV_geometry_shader_passthrough

Fix a large number of Clang warnings about inconsistent usage of 'override'

Also, eliminate the 'atom' field of TPpToken.

Parsing a real 300 line shader, through to making the AST, is about 10% faster.

Memory is slightly reduced (< 1%).

The whole google-test suite, inclusive of all testing overhead, SPIR-V generation,
etc., runs 3% faster.

Since this is a code *simplification* that leads to perf. improvement, I'm not
going to invest too much more in measuring the perf. than this. The PP code is
simply now in a better state to see how to further rationalize/improve it.

Always keep 'token' outside.
Always return the string to upper levels inside.

CMake: Add option to make it possible to not build the executables

Add support for SPV_NV_sample_mask_override_coverage

Removed the preprocesser memory pool.

Removed extra copies and unnecessary allocations of objects related to the ones
that were using the pool.

Replaced some allocated pointers with objects instead, generally using more
modern techiques. There end up being fewer memory allocations/deletions to get right.

Overall combined effect of all changes is to use slightly less memory and
run slightly faster (< 1% for both, but noticable).

As part of simplifying the code base, this change makes it easier to see
PP symbol tracking, which I suspect has an even bigger run-time simplification
to make.

Also, checking both 'atom' and 'name' is redundant, and I'm hoping to
eliminate more atom stuff.

PP: Implement token pasting for PP identifiers.

WIP: Remapper: remove debug info for IDs stripped in other passes

HLSL: allow "sample" in expressions.

Unbreak build on FreeBSD with GCC/libstdc++ 6.2.0

Implement token pasting as per the C++ specification, within the current
style of the PP code.
Non-identifiers (turning 12 ## 10 into the numeral 1210) is not yet covered;
they should be a simple incremental change built on this one.
Addresses issue #255.

Unlike other qualifiers, HLSL allows "sample" to be either a qualifier keyword or an
identifier (e.g, a variable or function name).

A fix to allow this was made a while ago, but that fix was insufficient when 'sample'
was used in an expression.  The problem was around the initial ambiguity between:

   sample float a; // "sample" is part of a fully specified type
and
   sample.xyz;     // sample is a keyword in a dot expression

Both start the same.  The "sample" was being accepted as a qualifier before enough
further parsing was done to determine we were not a declaration after all.  This
consumed the token, causing it to fail for its real purpose.

Now, when accepting a fully specified type, the token is pushed back onto the stack if
the thing is not a fully specified type.  This leaves it available for subsequent
purposes.

Changed the "hlsl.identifier.sample.frag" test to exercise this situation, distilled
down from a production shaders.

Change unicode dash to ASCII.

This change is helpful for integration with Chromium, which recently
added a compiler option to warn when compiling any source files which
use extended characters. In this case the offending character was a
single unicode dash in a comment.