[ubsan] Turn on 'Has CXXABI' to enable support for -fsanitize=vptr on Darwin

[vsk]

Summary

-fsanitize=vptr doesn't work on Darwin because we don't compile the necessary sanitizer handlers.

Fix the issue by exposing the definition of SANITIZER_MIN_OSX_VERSION to the logic which sets "use_cxxabi_default". Once this definition is available, we can verify that the user's version of macOS supports -fsanitize=vptr and then turn it on.

[vsk]

Ping.

[beanz]

LGTM!

compiler-rt/trunk/CMakeLists.txt

 endif()
 
 option(COMPILER_RT_DEBUG "Build runtimes with full debug info" OFF)
 option(COMPILER_RT_EXTERNALIZE_DEBUGINFO
   "Generate dSYM files and strip executables and libraries (Darwin Only)" OFF)
 # COMPILER_RT_DEBUG_PYBOOL is used by lit.common.configured.in.
 pythonize_bool(COMPILER_RT_DEBUG)
 
+include(config-ix)
+
 if(APPLE AND SANITIZER_MIN_OSX_VERSION VERSION_LESS "10.9")
   # Mac OS X prior to 10.9 had problems with exporting symbols from
   # libc++/libc++abi.
   set(use_cxxabi_default OFF)
 elseif(MSVC)
   set(use_cxxabi_default OFF)
 else()
   set(use_cxxabi_default ON)
 endif()
 
 option(SANITIZER_CAN_USE_CXXABI "Sanitizers can use cxxabi" ${use_cxxabi_default})
 pythonize_bool(SANITIZER_CAN_USE_CXXABI)
 
 #================================
 # Setup Compiler Flags
 #================================
 
-include(config-ix)
-
 if(MSVC)
   # Override any existing /W flags with /W4. This is what LLVM does.  Failing to
   # remove other /W[0-4] flags will result in a warning about overriding a
   # previous flag.
   if (COMPILER_RT_HAS_W4_FLAG)
     string(REGEX REPLACE " /W[0-4]" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
     string(REGEX REPLACE " /W[0-4]" "" CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS}")
     append_string_if(COMPILER_RT_HAS_W4_FLAG /W4 CMAKE_C_FLAGS CMAKE_CXX_FLAGS)