[clang-tools-extra] Adding Separate OpenMP Offloading Backend to `libcxx/include/__algorithm/pstl_backends` (PR #66968)

[Anton Rydahl via cfe-commits]

AntonRydahl wrote:

When adding the logic to error when if OpenMP is not enabled, I had to add a bit of CMake logic to allow installing the library without getting the error. On the good side, this also made it possible to enable the OpenMP toolchain when running the LIT tests, if the OpenMP backend was selected.

I now found that a few of the LIT tests failed, because I had not thought of handling cases where the base type of the iterator is not trivially copyable. For instance, some of the LIT tests may throw exceptions in the copy constructor. The commit I pushed today makes those tests pass while still allowing my local tests to be executed on the GPU.

What do you think of this change?

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