[llvm] [runtimes] Append `-nostd*++` flags only if necessary (PR #151930)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

I think it was added because originally, LLVM_ENABLE_RUNTIMES was invented for libc++(abi). It is C++, but obviously cannot link against libc++.so before libc++.so is built. For any library that is not libc++ but uses the C++ standard library (flang-rt, hwloc through openmp, maybe others), as long as there is a libc++/libstdc++ installed on the system and found by Clang, it can use that.

If we want those runtimes to link together with just-built libc++, we need a 4-stage bootstrapping build (clang -> compiler-rt builtins -> libc++ -> flang-rt/openmp). Or, at least this seems to be the current approach, make the configure step not depend on the existence of the C++ standard library, but for build-time, one can add a dependency to libc++ if built in the same runtimes build.

Also see #131010

Note that you can temporarily get rid of the flags using [`cmake_push_check_state(RESET)`]().

Also consider that you can add flags only for a specific language: `$<$<COMPILE_LANGUAGE:C>:-nostdlib++>`. Haven't tries whether this also works for `CMAKE_REQUIRED_FLAGS`.

https://github.com/llvm/llvm-project/pull/151930