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.

hlsl/hlslTokens.h

@@ -70,6 +70,7 @@ enum EHlslTokenClass {
     EHTokBuffer,
     EHTokVector,
     EHTokMatrix,
+    EHTokString,
 
     // scalar types
     EHTokVoid,