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/hlslGrammar.h

@@ -111,6 +111,7 @@ namespace glslang {
         bool acceptDefaultLabel(TIntermNode*&);
         void acceptArraySpecifier(TArraySizes*&);
         void acceptPostDecls(TQualifier&);
+        bool acceptDefaultParameterDeclaration(const TType&, TIntermTyped*&);
 
         HlslParseContext& parseContext;  // state of parsing and helper functions for building the intermediate
         TIntermediate& intermediate;     // the final product, the intermediate representation, includes the AST