buffer generated for the current translation unit. If they are
different, complain and then ignore the PCH file. This effectively
checks for all compilation options that somehow would affect
preprocessor state (-D, -U, -include, the dreaded -imacros, etc.).

When we do accept the PCH file, throw away the contents of the
predefines buffer rather than parsing them, since all of the results
of that parsing are already stored in the PCH file. This eliminates
the ugliness with the redefinition of __builtin_va_list, among other
things.

llvm-svn: 68838