[clang] Generate empty .clang-format-ignore before running tests (PR #136154)

[Arthur Eubanks via cfe-commits]

aeubanks wrote:

ah yes, the build directory can be outside of the source directory, lgtm. still good to keep the other PR though so outside .clang-format-ignore files don't leak into clang-format used inside llvm-project

https://github.com/llvm/llvm-project/pull/136154