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.

hlsl/hlslParseables.cpp

@@ -586,7 +586,7 @@ void TBuiltInParseablesHlsl::initialize(int /*version*/, EProfile /*profile*/, c
         { "DeviceMemoryBarrier",              nullptr, nullptr,   "-",              "-",             EShLangPSCS },
         { "DeviceMemoryBarrierWithGroupSync", nullptr, nullptr,   "-",              "-",             EShLangCS },
         { "distance",                         "S",     "F",       "V,",             "F,",            EShLangAll },
-        { "dot",                              "S",     nullptr,   "V,",             "FI,",           EShLangAll },
+        { "dot",                              "S",     nullptr,   "SV,",            "FI,",           EShLangAll },
         { "dst",                              nullptr, nullptr,   "V4,",            "F,",            EShLangAll },
         // { "errorf",                           "-",     "-",       "",             "",             EShLangAll }, TODO: varargs
         { "EvaluateAttributeAtCentroid",      nullptr, nullptr,   "SVM",            "F",             EShLangPS },