[CFI] Diagnose when we CFI in diagnostic mode is unavailable on a toolchain.

Summary:
Namely, we must have proper C++ABI support in UBSan runtime. We don't
have a good way to check for that, so just assume that C++ABI support is
there whenever -fsanitize=vptr is supported (i.e. only on handful of
platforms).

Exact diagnostic is also tricky. It's not "cfi" that is unsupported,
just the diagnostic mode. So, I suggest to report that
"-fno-sanitize-trap=cfi-foobar" is incompatible with a given target
toolchain.

Test Plan: regression test suite

Reviewers: pcc

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D10751

llvm-svn: 240716