[libcxx] [libcxxabi] [llvm] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Johannes Doerfert via llvm-commits]

jdoerfert wrote:

I get this error because the host pstl is included.
Following your steps and adding -H to the clang call shows:
```
  .... /opt/rh/gcc-toolset-12/root/usr/lib/gcc/x86_64-redhat-linux/12/../../../../include/c++/12/pstl/parallel_backend_utils.h
  .... /opt/rh/gcc-toolset-12/root/usr/lib/gcc/x86_64-redhat-linux/12/../../../../include/c++/12/pstl/parallel_backend.h
  ..... /opt/rh/gcc-toolset-12/root/usr/lib/gcc/x86_64-redhat-linux/12/../../../../include/c++/12/pstl/parallel_backend_tbb.h
  ...... /usr/include/tbb/blocked_range.h
  ....... /usr/include/tbb/tbb_stddef.h
  ........ /usr/include/tbb/tbb_config.h
```
Adding `-stdlib=libc++` (and potentially the rpath /lib/x86_64-unknown-linux-gnu/) makes it compile fine. It crashes at runtime due to a missing symbol, but it compiles fine.

https://github.com/llvm/llvm-project/pull/66968