Clang API function terminology got changed

...at least in trunk 200400 towards Clang 3.5.

Change-Id: I6e295e3a4cf721fbda9df8e7c5bed3993ee78216
clang::DiagnosticsEnginge::getCustomDiagID now needs a literal FormatString

...at least in trunk 200400 towards Clang 3.5.

Change-Id: Ibe956a7f28c16510506bd354582b3fe5a72582e5