HLSL: fix GS implementation for EP wrapping

The prior implementation of GS did not work with the new EP wrapping architecture.
This fixes it: the Append() method now looks up the actual output rather
than the internal sanitized temporary type, and writes to that.

glslang/Include/BaseTypes.h

@@ -219,6 +219,7 @@ enum TBuiltInVariable {
     EbvFragDepthGreater,
     EbvFragDepthLesser,
     EbvStencilRef,
+    EbvGsOutputStream,
 
     EbvLast
 };