[clang-tools-extra] [run-clang-tidy, clang-tidy-diff] Accept directory as value for -export-fixes (PR #69453)

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dyung wrote:

Hi @amgebauer, not sure if you got the notification, but your change seems to be causing a test failure on two build bots, any idea why?
https://lab.llvm.org/buildbot/#/builders/139/builds/51852
https://lab.llvm.org/buildbot/#/builders/247/builds/10286

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