[UBSan] Provide default blacklist filename for UBSan

[shenhan]

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.

[vsk]

This won't do the right thing if more than one sanitizer with a default blacklist is enabled. It's also problematic that a default blacklist for one sanitizer can blacklist code for a different sanitizer. See:

https://reviews.llvm.org/D32043
https://reviews.llvm.org/D32047
https://reviews.llvm.org/D32842

IMO I think D32842 is the right path forward. I don't have the time to get back to it this week, but if you'd like to pick it up and respond to review feedback from @eugenis, please do (I can certainly help with the review).

[shenhan]

Thanks for pointing out the relevant CLs. I agree that's the clearer and better solution.

(The only concern is that it has been sitting there for 2 months. I'll check back.)

In D35849#820473, @vsk wrote:

This won't do the right thing if more than one sanitizer with a default blacklist is enabled. It's also problematic that a default blacklist for one sanitizer can blacklist code for a different sanitizer. See:

https://reviews.llvm.org/D32043
https://reviews.llvm.org/D32047
https://reviews.llvm.org/D32842

IMO I think D32842 is the right path forward. I don't have the time to get back to it this week, but if you'd like to pick it up and respond to review feedback from @eugenis, please do (I can certainly help with the review).

[vsk]

This lgtm, we shouldn't defer this until D32842 is done, as that may take a while.

lib/Driver/SanitizerArgs.cpp

 static bool getDefaultBlacklist(const Driver &D, SanitizerMask Kinds,
                                 std::string &BLPath) {
   const char *BlacklistFile = nullptr;
   if (Kinds & Address)
     BlacklistFile = "asan_blacklist.txt";
   else if (Kinds & Memory)
     BlacklistFile = "msan_blacklist.txt";
   else if (Kinds & Thread)
     BlacklistFile = "tsan_blacklist.txt";
   else if (Kinds & DataFlow)
     BlacklistFile = "dfsan_abilist.txt";
   else if (Kinds & CFI)
     BlacklistFile = "cfi_blacklist.txt";
+  else if (Kinds & Undefined)
+    BlacklistFile = "ubsan_blacklist.txt";
 
   if (BlacklistFile) {
     clang::SmallString<64> Path(D.ResourceDir);
     llvm::sys::path::append(Path, BlacklistFile);
     BLPath = Path.str();
     return true;
   }
   return false;
 }
 
 /// Sets group bits for every group that has at least one representative already
 /// enabled in \p Kinds.
 static SanitizerMask setGroupBits(SanitizerMask Kinds) {