[compiler-rt][Darwin] Fix linker errors for check-asan

[yln]

Summary

A recent change broke ninja check-asan on Darwin by causing an error
during linking of ASan unit tests [1].

Move the addition of -ObjC compiler flag outside of the new
if(COMPILER_RT_STANDALONE_BUILD) block. It doesn't add any global
flags (e.g, ${CMAKE_CXX_FLAGS}) and the decision to add is based
solely on source paths (${source_rpath}).

[1] 8b2fcc42b895, https://reviews.llvm.org/D84466

[aeubanks]

sorry for the breakage, thanks for the fix

[yln]

Thanks for the quick review! :)

compiler-rt/cmake/Modules/CompilerRTCompile.cmake

if(NOT COMPILER_RT_STANDALONE_BUILD)
     list(APPEND SOURCE_DEPS clang compiler-rt-headers)
   endif()
   if (TARGET CompilerRTUnitTestCheckCxx)
     list(APPEND SOURCE_DEPS CompilerRTUnitTestCheckCxx)
   endif()
   if(COMPILER_RT_STANDALONE_BUILD)
     # Only add global flags in standalone build.
     string(REGEX MATCH "[.](cc|cpp)$" is_cxx ${source_rpath})
-    string(REGEX MATCH "[.](m|mm)$" is_objc ${source_rpath})
     if(is_cxx)
       string(REPLACE " " ";" global_flags "${CMAKE_CXX_FLAGS}")
     else()
       string(REPLACE " " ";" global_flags "${CMAKE_C_FLAGS}")
     endif()
 
     if (MSVC)
       translate_msvc_cflags(global_flags "${global_flags}")
     endif()
 
     if (APPLE)
       set(global_flags ${OSX_SYSROOT_FLAG} ${global_flags})
     endif()
-    if (is_objc)
-      list(APPEND global_flags -ObjC)
-    endif()
 
     # Ignore unknown warnings. CMAKE_CXX_FLAGS may contain GCC-specific options
     # which are not supported by Clang.
     list(APPEND global_flags -Wno-unknown-warning-option)
     set(compile_flags ${global_flags} ${SOURCE_CFLAGS})
   else()
     set(compile_flags ${SOURCE_CFLAGS})
   endif()
+
+  string(REGEX MATCH "[.](m|mm)$" is_objc ${source_rpath})
+  if (is_objc)
+    list(APPEND compile_flags "-ObjC")
+  endif()
+
   add_custom_command(
     OUTPUT ${object_file}
     COMMAND ${COMPILER_RT_TEST_COMPILER} ${compile_flags} -c
             -o "${object_file}"
             ${source_rpath}
     MAIN_DEPENDENCY ${source}
     DEPENDS ${SOURCE_DEPS})
 endfunction()