Add names for composite constants in SPIR-V

Consider the following code:

    layout(constant_id=0) const int Y = 1;
    layout(constant_id=1) const int Z = 2;
    layout(constant_id=3) const int X = Y + Z;

Previously, it would produce SPIR-V decorations like this:

    Decorate 21(Y) SpecId 1
    Decorate 22 SpecId 3
    Decorate 33(Z) SpecId 0

This seems inaccurate, since the spec constant `X` that is dependent on
the two others did not get a name in the SPIR-V decorations. This behavior
may potentially negatively affect shader introspection capabilities.

This change alters the behavior to always add a name, which results in the code
above producing the following decorations:

    Decorate 21(Y) SpecId 1
    Decorate 22(X) SpecId 3
    Decorate 33(Z) SpecId 0