[UBSan] Optionally report summary in UBSan error reports.

By default summary is not printed if UBSan is run in a standalone mode,
but is printed if it's combined with another sanitizer (like ASan).


git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@218135 91177308-0d34-0410-b5e6-96231b3b80d8