Implement diagnostic mode for -fsanitize=cfi*, -fsanitize=cfi-diag.

This causes programs compiled with this flag to print a diagnostic when
a control flow integrity check fails instead of aborting. Diagnostics are
printed using UBSan's runtime library.

The main motivation of this feature over -fsanitize=vptr is fidelity with
the -fsanitize=cfi implementation: the diagnostics are printed under exactly
the same conditions as those which would cause -fsanitize=cfi to abort the
program. This means that the same restrictions apply regarding compiling
all translation units with -fsanitize=cfi, cross-DSO virtual calls are
forbidden, etc.

Differential Revision: http://reviews.llvm.org/D10268

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240109 91177308-0d34-0410-b5e6-96231b3b80d8