[LLVM] Disable clang-format for TableGen files (#177002)
Clang-format is not very maintained for TableGen, and seems to make odd
choices that differ significantly from how humans write and read the
backend's tablegen.
For the moment, disable clang-format for TableGen files. This should
also apply when using `git clang-format`, which should help with newer
contributors who sometimes end up formatting td files by accident.
