[Driver] Add support for default UBSan blacklists

[vsk]

Summary

Clang should look for a default UBSan blacklist in its resource dir.

Depends on D32043.

[filcab]

Should we simply not have ubsan_blacklist.txt if it's empty?

Otherwise LGTM

[vsk]

I'd like to address the current problems with sanitizer blacklists before moving on to this patch:
https://reviews.llvm.org/D32842

The summary is that the entries in a blacklist file apply to all sanitizers, resulting in potential false negatives when multiple default blacklists are loaded.

lib/Driver/SanitizerArgs.cpp

 /// Add default blacklists from the resource directory.
 static void findDefaultBlacklists(const Driver &D, SanitizerMask Kinds,
                                   std::vector<std::string> &BlacklistFiles) {
   const std::pair<SanitizerMask, const char *> DefaultBlacklists[] = {
       {Address, "asan_blacklist.txt"},
       {Memory, "msan_blacklist.txt"},
       {Thread, "tsan_blacklist.txt"},
       {DataFlow, "dfsan_abilist.txt"},
-      {CFI, "cfi_blacklist.txt"}};
+      {CFI, "cfi_blacklist.txt"},
+      {Undefined | Integer | Nullability, "ubsan_blacklist.txt"}};
   for (const auto &BL : DefaultBlacklists) {
     if (!(Kinds & BL.first))
       continue;
 
     clang::SmallString<64> Path(D.ResourceDir);
     llvm::sys::path::append(Path, BL.second);
     if (llvm::sys::fs::exists(Path))
       BlacklistFiles.push_back(Path.str());

test/Driver/fsanitize-blacklist.c

 // CHECK-DEFAULT-BLACKLIST: -fsanitize-blacklist={{.*}}asan_blacklist.txt
 
 // Check that default blacklists are not added unless the matching sanitizer is
 // enabled, even if the blacklist exists.
 // RUN: FileCheck %s --implicit-check-not=cfi_blacklist.txt -input-file %t.cc1_asan
 
 // Check that we can add multiple default blacklists if the matching sanitizers
 // are enabled.
-// RUN: %clang -target x86_64-linux-gnu -fsanitize=address,cfi -flto -fvisibility=hidden -resource-dir=%S/Inputs/resource_dir %s -### 2>&1 | FileCheck %s --check-prefix=MULTIPLE-DEFAULT-BLACKLISTS
+// RUN: %clang -target x86_64-linux-gnu -fsanitize=address,cfi,undefined -flto -fvisibility=hidden -resource-dir=%S/Inputs/resource_dir %s -### 2>&1 | FileCheck %s --check-prefix=MULTIPLE-DEFAULT-BLACKLISTS
 // MULTIPLE-DEFAULT-BLACKLISTS-DAG: -fsanitize-blacklist={{.*}}asan_blacklist.txt
 // MULTIPLE-DEFAULT-BLACKLISTS-DAG: -fsanitize-blacklist={{.*}}cfi_blacklist.txt
+// MULTIPLE-DEFAULT-BLACKLISTS-DAG: -fsanitize-blacklist={{.*}}ubsan_blacklist.txt
 
 // Ignore -fsanitize-blacklist flag if there is no -fsanitize flag.
 // RUN: %clang -target x86_64-linux-gnu -fsanitize-blacklist=%t.good %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-SANITIZE --check-prefix=DELIMITERS
 // CHECK-NO-SANITIZE-NOT: -fsanitize-blacklist
 
 // Ignore -fsanitize-blacklist flag if there is no -fsanitize flag.
 // Now, check for the absense of -fdepfile-entry flags.
 // RUN: %clang -target x86_64-linux-gnu -fsanitize-blacklist=%t.good %s -### 2>&1 | FileCheck %s --check-prefix=CHECK-NO-SANITIZE2 --check-prefix=DELIMITERS