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.

Test/hlsl.string.frag

@@ -0,0 +1,12 @@
+string s = "string1";
+string e = "";
+string bracket < string a = "nested" ; > ; 
+string brackets < string b = "nest1" ; string c = "nest2" ; string d = "nest3" ; > ;
+string brackete1 <  > ;
+string brackete2 < ; > ;
+string brackete3 < ; ; > ;
+
+float main(float f)
+{
+    return f;
+}