[CMake] Use cxx-headers as a depedency for C++ headers

[phosek]

Summary

We no longer pass CLANG_DEFAULT_CXX_STDLIB to the runtimes build
as it was causing issues so we can no longer use this variable. We
instead use cxx-headers as a dependency whenever this is available
since both XRay and libFuzzer are built as static libraries so this
is sufficient.

[dberris]

LGTM on the XRay side.

compiler-rt/lib/fuzzer/CMakeLists.txt

CHECK_CXX_SOURCE_COMPILES("
   " HAS_THREAD_LOCAL)
 
 set(LIBFUZZER_CFLAGS ${SANITIZER_COMMON_CFLAGS})
 
 if(OS_NAME MATCHES "Linux|Fuchsia" AND COMPILER_RT_LIBCXX_PATH)
   list(APPEND LIBFUZZER_CFLAGS -nostdinc++ -D_LIBCPP_ABI_VERSION=Fuzzer)
   # Remove -stdlib= which is unused when passing -nostdinc++.
   string(REGEX REPLACE "-stdlib=[a-zA-Z+]*" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})
-elseif(CLANG_DEFAULT_CXX_STDLIB STREQUAL "libc++" AND (TARGET cxx OR HAVE_LIBCXX))
-  set(LIBFUZZER_DEPS cxx)
+elseif(TARGET cxx-headers OR HAVE_LIBCXX)
+  set(LIBFUZZER_DEPS cxx-headers)
 endif()
 
 append_list_if(COMPILER_RT_HAS_OMIT_FRAME_POINTER_FLAG -fno-omit-frame-pointer LIBFUZZER_CFLAGS)
 
 if (CMAKE_CXX_FLAGS MATCHES "fsanitize-coverage")
   list(APPEND LIBFUZZER_CFLAGS -fno-sanitize-coverage=trace-pc-guard,edge,trace-cmp,indirect-calls,8bit-counters)
 endif()
 

compiler-rt/lib/xray/CMakeLists.txt

append_list_if(
   COMPILER_RT_BUILD_XRAY_NO_PREINIT XRAY_NO_PREINIT XRAY_COMMON_DEFINITIONS)
 
 add_compiler_rt_component(xray)
 
 set(XRAY_COMMON_RUNTIME_OBJECT_LIBS
     RTSanitizerCommon
     RTSanitizerCommonLibc)
 
-if (CLANG_DEFAULT_CXX_STDLIB STREQUAL "libc++" AND (TARGET cxx OR HAVE_LIBCXX))
-  set(XRAY_DEPS cxx)
+if (TARGET cxx-headers OR HAVE_LIBCXX)
+  set(XRAY_DEPS cxx-headers)
 endif()
 
 if (APPLE)
   set(XRAY_LINK_LIBS ${SANITIZER_COMMON_LINK_LIBS})
   add_asm_sources(XRAY_ASM_SOURCES xray_trampoline_x86_64.S)
 
   add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINK_FLAGS)
   add_weak_symbols("xray" WEAK_SYMBOL_LINK_FLAGS)