HLSL default function parameters

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.

gtests/Hlsl.FromFile.cpp

@@ -160,6 +160,8 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.numericsuffixes.frag", "main"},
         {"hlsl.numthreads.comp", "main_aux1"},
         {"hlsl.overload.frag", "PixelShaderFunction"},
+        {"hlsl.params.default.frag", "main"},
+        {"hlsl.params.default.negative.frag", "main"},
         {"hlsl.partialInit.frag", "PixelShaderFunction"},
         {"hlsl.pp.line.frag", "main"},
         {"hlsl.precise.frag", "main"},