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.

glslang/MachineIndependent/SymbolTable.h

@@ -191,6 +191,7 @@ protected:
 struct TParameter {
     TString *name;
     TType* type;
+    TIntermTyped* defaultValue;
     void copyParam(const TParameter& param) 
     {
         if (param.name)
@@ -198,6 +199,7 @@ struct TParameter {
         else
             name = 0;
         type = param.type->clone();
+        defaultValue = param.defaultValue;
     }
 };
 
@@ -209,12 +211,12 @@ public:
     explicit TFunction(TOperator o) :
         TSymbol(0),
         op(o),
-        defined(false), prototyped(false) { }
+        defined(false), prototyped(false), defaultParamCount(0) { }
     TFunction(const TString *name, const TType& retType, TOperator tOp = EOpNull) :
         TSymbol(name),
         mangledName(*name + '('),
         op(tOp),
-        defined(false), prototyped(false) { returnType.shallowCopy(retType); }
+        defined(false), prototyped(false), defaultParamCount(0) { returnType.shallowCopy(retType); }
     virtual TFunction* clone() const;
     virtual ~TFunction();
 
@@ -226,6 +228,9 @@ public:
         assert(writable);
         parameters.push_back(p);
         p.type->appendMangledName(mangledName);
+
+        if (p.defaultValue != nullptr)
+            defaultParamCount++;
     }
 
     virtual const TString& getMangledName() const { return mangledName; }
@@ -238,7 +243,13 @@ public:
     virtual void setPrototyped() { assert(writable); prototyped = true; }
     virtual bool isPrototyped() const { return prototyped; }
 
+    // Return total number of parameters
     virtual int getParamCount() const { return static_cast<int>(parameters.size()); }
+    // Return number of parameters with default values.
+    virtual int getDefaultParamCount() const { return defaultParamCount; }
+    // Return number of fixed parameters (without default values)
+    virtual int getFixedParamCount() const { return getParamCount() - getDefaultParamCount(); }
+
     virtual TParameter& operator[](int i) { assert(writable); return parameters[i]; }
     virtual const TParameter& operator[](int i) const { return parameters[i]; }
 
@@ -255,6 +266,7 @@ protected:
     TOperator op;
     bool defined;
     bool prototyped;
+    int  defaultParamCount;
 };
 
 //