[UBSan] Add silence_unsigned_overflow flag.

Summary:
Setting UBSAN_OPTIONS=silence_unsigned_overflow=1 will silence all UIO
reports.  This feature, combined with
-fsanitize-recover=unsigned-integer-overflow, is useful for providing
fuzzing signal without the excessive log output.

Helps with https://github.com/google/oss-fuzz/issues/910.

Reviewers: kcc, vsk

Reviewed By: vsk

Subscribers: vsk, kubamracek, Dor1s, llvm-commits

Differential Revision: https://reviews.llvm.org/D48660

llvm-svn: 335762