[clang] [clang] Refactor clang's keyword enable/disable mechanism to allow lldb to re-use it (PR #165323)

[Michael Buch via cfe-commits]

https://github.com/Michael137 approved this pull request.

LGTM, thanks for splitting this out! Could now even be unit-testable (not that you should do that as part of this PR though). Please wait a couple of days for the Clang maintainers to approve before merging

https://github.com/llvm/llvm-project/pull/165323