WIP: HLSL: Support InputPatch variables in patch constant functions

Previously, patch constant functions only accepted OutputPatch.  This
adds InputPatch support, via a pseudo-builtin variable type, so that
the patch can be tracked clear through from the qualifier.

glslang/Include/BaseTypes.h

@@ -220,6 +220,8 @@ enum TBuiltInVariable {
     EbvFragDepthLesser,
     EbvStencilRef,
     EbvGsOutputStream,
+    EbvOutputPatch,
+    EbvInputPatch,
 
     EbvLast
 };