[clang-tools-extra] [clang-include-cleaner] Add --mapping-file support for IWYU .imp mapping files. (PR #195987)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

clang-include-cleaner doesn't really have much active maintenance for it, but it's on my plate for review as lead for clang-tools-extra. I gave the PR as good of a review as I could and did not spot much, but more eyeballs on the changes would be appreciated.

CC @dmaclach and @nico as folks who have done some recent changes and reviews here

https://github.com/llvm/llvm-project/pull/195987