[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

llvm-svn: 367478