[compiler-rt][UBSan] silence_unsigned_overflow: do *NOT* ignore *fatal* unsigned overflows

Summary:
D48660 / rL335762 added a `silence_unsigned_overflow` env flag for [[ https://github.com/google/oss-fuzz/pull/1717 | oss-fuzz needs ]],
that allows to silence the reports from unsigned overflows.
It makes sense, it is there because `-fsanitize=integer` sanitizer is not enabled on oss-fuzz,
so this allows to still use it as an interestingness signal, without getting the actual reports.

However there is a slight problem here.
All types of unsigned overflows are ignored.
Even if `-fno-sanitize-recover=unsigned` was used (which means the program will die after the report)
there will still be no report, the program will just silently die.

At the moment there are just two projects on oss-fuzz that care:
* [[ https://github.com/google/oss-fuzz/blob/8eeffa627f937040aaf8ba1b7d93f43f77d74fb9/projects/llvm_libcxx/build.sh#L18-L20 | libc++ ]]
* [[ https://github.com/google/oss-fuzz/blob/8eeffa627f937040aaf8ba1b7d93f43f77d74fb9/projects/librawspeed/build.sh | RawSpeed ]] (me)

I suppose this could be overridden there ^, but i really don't think this is intended behavior in any case..

Reviewers: kcc, Dor1s, #sanitizers, filcab, vsk, kubamracek

Reviewed By: Dor1s

Subscribers: dberris, mclow.lists, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D54771

llvm-svn: 347415