[NFC][clang] Refactor getCompilationPhases()+Types.def step 3.
Second landing attempt: Changed TY_ObjCXXHeader to TY_PP_ObjCXXHeader to fix
-xobjective-c++-header. This time I verified against preprocessor output.
Dropping the 'u' entry and the entire Flags table from Types.def.
Now it'll be a bit easier to tablegenify this.
Differential Revision: https://reviews.llvm.org/D65308