[clang] [clang-format] Add --fallback-style to git-clang-format (PR #137609)

[Andrej Pištek via cfe-commits]

apdofficial wrote:

@mydeveloperday, that is a valid question. 

The reason is that we have many legacy repositories that do not have a `.clang-format file`. The legacy repositories are in a maintenance phase, and most importantly, they follow a custom code style. We want to implement a standard code style without refactoring the legacy code. This means adding a `.clang-format` file would not truthfully reflect the actual code style in the legacy repositories. This MR enables checking only the newly added code according to the new code style.

https://github.com/llvm/llvm-project/pull/137609