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.

Test/hlsl.params.default.negative.frag

@@ -0,0 +1,50 @@
+uniform int4 ui4;
+uniform float ufvar;
+
+static const int cia = -4;
+static const int cib = -42;
+
+int4 fn1(int4 p0) { return int4(1,2,3,4); }
+
+int4 fn1(int4 p0, bool b1, bool b2 = false) {
+    return p0;
+}
+
+int4 fn1(int4 p0,
+         int4 p1 : FOO = int4(-1,-2,-3, cia),
+         int p2[2] : BAR = { int(1), 2 },
+         int p3 = abs(cib) )
+{
+    return p0 + p1 + p2[0] + p3;
+}
+
+// These should not be ambiguous if given either an int or a float explicit second parameter.
+int4 fn2(int4 p0, int x = 3)
+{
+    return int4(10,11,12,13);
+}
+
+int4 fn2(int4 p0, float x = ufvar) // ERROR: non-const expression
+{
+    return p0 + int4(20,21,22,23);
+}
+
+void fn3(int p0 = 5, int p1)  // ERROR no-default param after default param
+{
+}
+
+int4 main() : SV_Target0
+{
+    int myarray[2] = {30,31};
+
+    return fn1(100) +                    // ERROR: ambiguous
+           fn1(101, ui4) +
+           fn1(102, ui4, myarray) +
+           fn1(103, ui4, myarray, 99) +
+           fn1(104, false) +
+           fn1(105, false, true) +
+
+           fn2(112) +                    // ERROR: ambiguous
+           fn2(110, 11.11) +             // calls int4, float form
+           fn2(111, 12);                 // calls int4, int form
+}