[flang-rt] Fix flang-rt unit test linking with LLVM_ENABLE_LIBCXX

[DavidTruby]

This patch fixes and issue where the flang-rt unit tests fail to link
when using LLVM_ENALBE_LIBCXX for the LLVM build (and not using
CLANG_DEFAULT_CXX_STDLIB=libc++) because the -DLLVM_ENABLE_LIBCXX is
not passed on to the flang-rt cmake command.
By adding it to the PASSTHROUGH flags it gets used when building the
runtime tests as well.

Note: this will also cause the runtimes themselves to be built with
libc++, but this is also most likely what the user wanted when passing
-DLLVM_ENABLE_LIBCXX, and was the behaviour before the runtimes were
split into flang-rt. It won't affect linkage for the runtimes as they
only use the header only parts of the C++ library.

[Meinersbur]

#134990 is better because it applies to projects that want to link to LLVM libraries, not just those that use the LLVM_ENABLE_RUNTIMES bootstrapping builds.

Had to be reverted because it cause buildbot failures and did not even fix flang-aarch64-libcxx (nor will this PR).

[pawosm-arm]

I've tried this patch, but still no luck:

openmp/runtime/src/kmp.h:80:10: fatal error: 'limits' file not found
   80 | #include <limits>
      |          ^~~~~~~~
1 error generated.

[Meinersbur]

Created PR for re-applying #134990: #139712

@pawosm-arm Should fix the error you are observing by not adding -stdlib=libc++ if those include headers are not found. However, mismatching C++ standard libraries is not a good idea.

[pawosm-arm]

Following @tblah suggestion I've reverted 77581e2 and it solved my problem.

[Meinersbur]

This PR assumes that flang-rt is built in a bootrapping built. When building flang-rt in a standard/standalone build using an LLVM that was built with LLVM_ENABLE_LIBCXX=ON, one would still get the same linking error.

[DavidTruby]

Closing in favour of #139712