WIP: HLSL: support global const initializers from non-constant rvalues

Semantic test left over from other source languages is removed, since this is permitted by HLSL.
Also, to support the functionality, a targeted test is performed for this case and it is
turned into a EvqGlobal qualifier to create an AST initialization segment when needed.

Constness is now propagated up aggregate chains during initializer construction.  This
handles hierarchical cases such as the distinction between:

    static const float2 a[2] = { { 1, 2 }, { 3, 4} };

vs

    static const float2 a[2] = { { 1, 2 }, { cbuffer_member, 4} };

The first of which can use a first class constant initalization, and the second cannot.

Test/hlsl.global-const-init.frag

@@ -0,0 +1,14 @@
+
+cbuffer CB {
+    float4 foo;
+};
+
+static const float4 bar = foo; // test const (in the immutable sense) initializer from non-const.
+
+static const float2 a1[2] = { { 1, 2 }, { foo.x, 4 } }; // not entirely constant
+static const float2 a2[2] = { { 5, 6 }, { 7, 8 } };     // entirely constant
+
+float4 main() : SV_Target0
+{
+    return bar;
+}