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.