HLSL: allow mixed user & builtin members in hull shader output structure

Hull shaders have an implicitly arrayed output.  This is handled by creating an arrayed form of the
provided output type, and writing to the element of it indexed by InvocationID.

The implicit indirection into that array was causing some troubles when copying to a split
structure.  handleAssign was able to handle simple symbol lvalues, but not an lvalue composed
of an indirection into an array.

SPIRV/GlslangToSpv.cpp

@@ -3555,7 +3555,7 @@ spv::Id TGlslangToSpvTraverser::handleUserFunctionCall(const glslang::TIntermAgg
     const glslang::TIntermSequence& glslangArgs = node->getSequence();
     const glslang::TQualifierList& qualifiers = node->getQualifierList();
 
-    // Encapsulate lvalue logic, used in two places below, for safety.
+    // Encapsulate lvalue logic, used in several places below, for safety.
     const auto isLValue = [](int qualifier, const glslang::TType& paramType) -> bool {
         return qualifier != glslang::EvqConstReadOnly || paramType.containsOpaque();
     };