After issuing a diagnostic for undefining or redefining a builtin macro,
continue parsing the directive rather than silently discarding it.

Allowing undef or redef of __TIME__ and __DATE__ is important to folks
who want stable, reproducible builds.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176540 91177308-0d34-0410-b5e6-96231b3b80d8