[UBSan] Provide default blacklist filename for UBSan

Summary:
This is to provide a default blacklist filename for UBSan.

While UBSan is turned on, it's better that clang pick up a blacklist file (when exists), just as what ASan / MSan does, so we do not end up adding the "-fsanitize-blacklist" option to every command line.

Reviewers: chandlerc, echristo, vsk, eugenis

Reviewed By: vsk, eugenis

Subscribers: vsk, eugenis, echristo, cfe-commits

Differential Revision: https://reviews.llvm.org/D35849

llvm-svn: 309873