[clang] [clang-format] Add ConfigFile option (PR #113864)

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xGeovanni wrote:

@owenca Hi, thanks for this PR. Arte you still planning to move forward with this? It would be helpful for my use case, wehre we have a common style across all projects except that some need some special header-ordering rules to work around library requirements, but other projects don't need those rules. They cannot be organised in a folder hierarchy that would make `InheritParentConfig` work for this use case. Thanks.

https://github.com/llvm/llvm-project/pull/113864