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.

Test/baseResults/hlsl.hull.ctrlpt-1.tesc.out

@@ -29,7 +29,7 @@ triangle order = cw
 0:?         'cpid' ( temp uint)
 0:?         'cpid' ( in uint InvocationID)
 0:27      move second child to first child ( temp structure{ temp 3-component vector of float val})
-0:27        indirect index ( temp structure{ temp 3-component vector of float val})
+0:27        indirect index (layout( location=0) out structure{ temp 3-component vector of float val})
 0:?           '@entryPointOutput' (layout( location=0) out 3-element array of structure{ temp 3-component vector of float val})
 0:?           'cpid' ( in uint InvocationID)
 0:27        Function Call: @main(struct-hs_in_t-vf31[3];u1; ( temp structure{ temp 3-component vector of float val})
@@ -229,7 +229,7 @@ triangle order = cw
 0:?         'cpid' ( temp uint)
 0:?         'cpid' ( in uint InvocationID)
 0:27      move second child to first child ( temp structure{ temp 3-component vector of float val})
-0:27        indirect index ( temp structure{ temp 3-component vector of float val})
+0:27        indirect index (layout( location=0) out structure{ temp 3-component vector of float val})
 0:?           '@entryPointOutput' (layout( location=0) out 3-element array of structure{ temp 3-component vector of float val})
 0:?           'cpid' ( in uint InvocationID)
 0:27        Function Call: @main(struct-hs_in_t-vf31[3];u1; ( temp structure{ temp 3-component vector of float val})