[clang-tools-extra] [run-clang-tidy] Add option -export-directory (PR #69453)

[Piotr Zegar via cfe-commits]

PiotrZSL wrote:

There is already --export_fixes that can be used in +- same way.
Maybe better would be to change --export_fixes to take filename or directory (ends with / or exist as directory), and in that case it could save to directory or merge to single file.

https://github.com/llvm/llvm-project/pull/69453