[CMake] Tell libc++ that we're using compiler-rt on Apple platforms

[ldionne]

Ping.

[smeenai]

Is this to work around https://github.com/llvm/llvm-project/blob/f84083b4dbb1ddb6d2783400f11121f490cdb5a8/libcxx/lib/CMakeLists.txt#L312?

[ldionne]

In D58921#1423294, @smeenai wrote:

Is this to work around https://github.com/llvm/llvm-project/blob/f84083b4dbb1ddb6d2783400f11121f490cdb5a8/libcxx/lib/CMakeLists.txt#L312?

Yes, precisely. Note that I think this option should be ON by default, but specifying this explicitly doesn't hurt regardless.

clang/cmake/caches/Apple-stage2.cmake

if(LLVM_ENABLE_LTO AND NOT LLVM_ENABLE_LTO STREQUAL "THIN")
   set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O2 -gline-tables-only -DNDEBUG" CACHE STRING "")
   set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -gline-tables-only -DNDEBUG" CACHE STRING "")
 endif()
 set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
 
 set(LIBCXX_INSTALL_LIBRARY OFF CACHE BOOL "")
 set(LIBCXX_INSTALL_HEADERS ON CACHE BOOL "")
 set(LIBCXX_INCLUDE_TESTS OFF CACHE BOOL "")
+set(LIBCXX_USE_COMPILER_RT ON CACHE BOOL "")
 set(LLVM_LTO_VERSION_OFFSET 3000 CACHE STRING "")
 
 # Generating Xcode toolchains is useful for developers wanting to build and use
 # clang without installing over existing tools.
 set(LLVM_CREATE_XCODE_TOOLCHAIN ON CACHE BOOL "")
 
 # setup toolchain
 set(LLVM_INSTALL_TOOLCHAIN_ONLY ON CACHE BOOL "")