HLSL: use prefix for builtin functions names to avoid namespace collisions

It would have been possible for globally scoped user functions to collide
with builtin method names.  This adds a prefix to avoid polluting the
namespace.

Ideally this would be an invalid character to use in user identifiers, but
as that requires changing the scanner, for the moment it's an unlikely yet
valid prefix.

hlsl/hlslParseHelper.h

@@ -390,6 +390,12 @@ protected:
     TVector<TString> currentTypePrefix;
 };
 
+// This is the prefix we use for builtin methods to avoid namespace collisions with
+// global scope user functions.
+// TODO: this would be better as a nonparseable character, but that would
+// require changing the scanner.
+#define BUILTIN_PREFIX "__BI_"
+
 } // end namespace glslang
 
 #endif // HLSL_PARSE_INCLUDED_