[SYCL][UR] Fix build with libc++ (#19521)
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](https://github.com/intel/llvm/blob/sycl/sycl/source/CMakeLists.txt#L98).
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.
https://github.com/intel/llvm/issues/19347
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Signed-off-by: Sarnie, Nick <nick.sarnie@intel.com>
