Create a portability branch that supports clang10 through clang18

[tpatki]

PR #167 will be cleaned up and converted to a new branch, that will reside under the main repository as a portability branch which is experimental.
This will support both the legacy and new pass manager, and users are expected to find it highly beneficial if they switch between different Clang versions. Supported Clang versions will be Clang-9 through Clang-18.

We do our best to test and update this branch, but we will not guarantee production-grade support.
Production-quality will be guaranteed on the main branch, which will be pinned to Clang v18 and will use the new pass manager.