[llvm] [OpenACC] Initial commits to support OpenACC (PR #70234)

[Erich Keane via llvm-commits]

erichkeane wrote:

Build failures are all 1 test, and are:

`File /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-fdgrz-1/llvm-project/clang-ci/libcxx/test/libcxx/algorithms/cpp17_iterator_concepts.verify.cpp Line 410: cannot find start of regex ('{{') in static assertion failed`

The patch responsible was reverted here: https://github.com/llvm/llvm-project/commit/fde1ecdec878be591479f508d6a373701deb137e

https://github.com/llvm/llvm-project/pull/70234