[SYCL][UR] Fix build with libc++

[sarnex]

We previously added support for building libsycl individually with libc++. At that time, libsycl didn't link against libLLVM so they did not need to be using the same STL implementation.

Now, libsycl does link against libLLVM, one example here.

Remove all the code that was to support libsycl in particular and just make it work with the normal option LLVM uses, LLVM_ENABLE_LIBCXX. Most of the code I added actually checks LLVM_LIBCXX_USED, because LLVM checks at configure time if libc++ actually works even when passed LLVM_ENABLE_LIBCXX, and if it fails, it falls back to libstdc++, and most of the time we actually want to know if we are actually using libc++.

I added a CMake option SYCL_LIBDEVICE_CXX_FLAGS to set the flags for libdevice because it will be required if someone doesn't use the LLVM option and just does it through CMAKE_CXX_FLAGS, as those aren't used for libdevice but it still uses the STL.

The two changes in unified-runtime/source/loader/layers/sanitizer/msan/msan_libdevice.hpp and sycl/source/builtins/common_functions.cpp fix compile errors with libc++.

I also plan to add a job to the nightly to lock this down.

#19347

[sarnex]

the change here is because --print-file-name doesn't work as required here so the lookup always fails, so just fall back to the underlyting compiler's errors.