Improved fix for buffer reference constants

This is an alternate fix for the issue described in commit be63facd, whose
solution didn't work if there were non-trivial operations involved in computing
a constant initializer which caused the 'constant unfolding' code to kick in
(addConstantReferenceConversion). Instead, this change does the 'unfolding'
later in createSpvConstantFromConstUnionArray. If a reference-type constant has
survived that long, then folding is already done, this must be a 'real' (inside
a function) use of the constant, and it should be safe to unfold and apply the
bitcast.

SPIRV/GlslangToSpv.cpp

@@ -7705,6 +7705,10 @@ spv::Id TGlslangToSpvTraverser::createSpvConstantFromConstUnionArray(const glsla
         case glslang::EbtBool:
             scalar = builder.makeBoolConstant(zero ? false : consts[nextConst].getBConst(), specConstant);
             break;
+        case glslang::EbtReference:
+            scalar = builder.makeUint64Constant(zero ? 0 : consts[nextConst].getU64Const(), specConstant);
+            scalar = builder.createUnaryOp(spv::OpBitcast, typeId, scalar);
+            break;
         default:
             assert(0);
             break;