[clang] [clang] Introduce copy-on-write `CompilerInvocation` (PR #65412)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

I evicted a couple of commits from this PR in order to work better with the new GitHub PR "squash and merge" workflow. I'll create new PRs for the remaining commits.

https://github.com/llvm/llvm-project/pull/65412