[clang] [clang-tools-extra] [llvm] [clang-format] Add support for BasedOnStyle referencing an arbitrary file (PR #107312)

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https://github.com/mydeveloperday requested changes to this pull request.

I'm with @HazardyKnusperkeks  you are changing too much in one go, get this established in clang-format first and then look to rolling out to other tools that use lib/format

YAML changes should be separate and agreed with that code owner we don't own that

https://github.com/llvm/llvm-project/pull/107312