[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 google/oss-fuzz#910.

Reviewers: kcc, vsk

Reviewed By: vsk

Subscribers: vsk, kubamracek, Dor1s, llvm-commits

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@335762 91177308-0d34-0410-b5e6-96231b3b80d8

docs/UndefinedBehaviorSanitizer.rst

@@ -180,6 +180,13 @@ will need to:
    ``UBSAN_OPTIONS=print_stacktrace=1``.
 #. Make sure ``llvm-symbolizer`` binary is in ``PATH``.
 
+Silencing Unsigned Integer Overflow
+===================================
+To silence reports from unsigned integer overflow, you can set
+``UBSAN_OPTIONS=silence_unsigned_overflow=1``.  This feature, combined with
+``-fsanitize-recover=unsigned-integer-overflow``, is particularly useful for
+providing fuzzing signal without blowing up logs.
+
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