RunClang: Refactor built-in predefines substitution
RunClang: Refactor built-in predefines substitution

Always split the predefines string into a prefix, body of actual
builtins, and suffix.  Replace the body with builtins detected from
--castxml-cc-<id>, if given.

CastXML/CastXML,bradking/CastXML,iMichka/CastXML,thewtex/CastXML,bradking/CastXML,iMichka/CastXML,thewtex/CastXML,thewtex/CastXML,ellert/CastXML,CastXML/CastXML,ellert/CastXML,ellert/CastXML,bradking/CastXML,CastXML/CastXML