[UBSan] Followup for silence_unsigned_overflow flag to handle negate overflows.

Summary:
That flag has been introduced in https://reviews.llvm.org/D48660 for
suppressing UIO error messages in an efficient way. The main motivation is to
be able to use UIO checks in builds used for fuzzing as it might provide an
interesting signal to a fuzzing engine such as libFuzzer.

See https://github.com/google/oss-fuzz/issues/910 for more information.

Reviewers: morehouse, kcc

Reviewed By: morehouse

Subscribers: kubamracek, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D49324

llvm-svn: 337068