[Sanitizers][Apple][Cmake] Lower C++ standard to 14 for single sanitizer test on Apple platforms

[thetruestblue]

Summary

This is a not-so-ideal suggestion on how we can add a flag to a single test within SanitizerUnitTestCommon tests. set_source_file_properties(<file> PROPERTIES COMPILE_FLAGS <flag>) does not seem to be considered in generate_compiler_rt_tests. Ideally, this flag would be added in the appropriate CMakeList.txt file in compiler-rt/lib/sanitizer_common/tests/. However, it was not clear how or if it would be possible to add a source-specific change in this file. There are examples of other compiler-rt test-suites using this (i.e. scudo), their source properties are considered (handled with add_compiler_rt_runtime), whereas these are not.

Alternatives:
-We could up this test's macos-min-version to 10.13, rather than lower C++ standard.
-We could apply this flag to the entire test-suite within CMakeList.txt.
-We could up the SANITIZER_MIN_VERSION_DEFAULT to 10.13

Can someone with more cmake know-how recommend an alternative?

rdar://98737270

[thetruestblue]

Abandoning for a more lightweight change.

compiler-rt/cmake/Modules/AddCompilerRT.cmake

 #                           DEPS <dependencies>
 #                           LINK_FLAGS <flags to use during linking>
 # )
 function(generate_compiler_rt_tests test_objects test_suite testname arch)
   cmake_parse_arguments(TEST "" "KIND;RUNTIME;SUBDIR"
     "SOURCES;COMPILE_DEPS;DEPS;CFLAGS;LINK_FLAGS" ${ARGN})
 
   foreach(source ${TEST_SOURCES})
+      if (${source} STREQUAL "sanitizer_allocator_test.cpp" AND APPLE)
+        # c++17 requires upping min version to 10.13. rdar://98737270
+        # Keep 14 standard in allocator test to maintain current back compatibility.
+        set(STD_CXX "-std=c++14")
+      endif()
       sanitizer_test_compile(
         "${test_objects}" "${source}" "${arch}"
         KIND ${TEST_KIND}
         COMPILE_DEPS ${TEST_COMPILE_DEPS}
         DEPS ${TEST_DEPS}
-      CFLAGS ${TEST_CFLAGS}
+        CFLAGS ${TEST_CFLAGS} ${STD_CXX}
       )
+      unset(STD_CXX)
   endforeach()
 
   set(TEST_DEPS ${${test_objects}})
 
   if(NOT "${TEST_RUNTIME}" STREQUAL "")
     list(APPEND TEST_DEPS ${TEST_RUNTIME})
     list(APPEND "${test_objects}" $<TARGET_FILE:${TEST_RUNTIME}>)
   endif()