Fix UBSan bots by blacklisting bits/stl_tree.h.

[krasin]

Summary

libstdc++ has some undefined behavior in bits/stl_tree.h that
has recently became excercised by some of the LLVM code.
Given that fixing libstdc++ will take years, adding the file
into a blacklist to fix bots seems like a necessity.

[krasin]

Remove leading space.

[krasin]

Add a comment to the suppression.

cmake/modules/HandleLLVMOptions.cmake

elseif (LLVM_USE_SANITIZER MATCHES "Memory(WithOrigins)?")
       append("-fsanitize=memory" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
       if(LLVM_USE_SANITIZER STREQUAL "MemoryWithOrigins")
         append("-fsanitize-memory-track-origins" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
       endif()
     elseif (LLVM_USE_SANITIZER STREQUAL "Undefined")
       append_common_sanitizer_flags()
       append("-fsanitize=undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all"
               CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+      append(" -fsanitize-blacklist=${CMAKE_SOURCE_DIR}/utils/sanitizers/ubsan_blacklist.txt"
+        CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     elseif (LLVM_USE_SANITIZER STREQUAL "Thread")
       append_common_sanitizer_flags()
       append("-fsanitize=thread" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
     elseif (LLVM_USE_SANITIZER STREQUAL "Address;Undefined" OR
             LLVM_USE_SANITIZER STREQUAL "Undefined;Address")
       append_common_sanitizer_flags()
       append("-fsanitize=address,undefined -fno-sanitize=vptr,function -fno-sanitize-recover=all"
               CMAKE_C_FLAGS CMAKE_CXX_FLAGS)

utils/sanitizers/ubsan_blacklist.txt

+# This blacklist should be applied when LLVM is built
+# with -fsanitize=undefined instrumentation. It exists
+# because libstdc++ has some undefined behavior issues
+# in some of the headers, in particular, stl_tree.h.
+src:*bits/stl_tree.h