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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

I've added the clangd maintainers given the relationship to https://github.com/clangd/clangd/issues/1276

Also CC @kimgr @bolshakov-a for IWYU perspective.

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