HLSL: Add string basic type and recognize string declaration grammar.

This includes the "< decl ; decl ; >" syntax which has its own namespace.
This functionality is not implemented, just silently accepted.

glslang/Include/BaseTypes.h

@@ -55,6 +55,10 @@ enum TBasicType {
     EbtSampler,
     EbtStruct,
     EbtBlock,
+
+    // HLSL types that live only temporarily.
+    EbtString,
+
     EbtNumTypes
 };