[llvm] [TableGen] Strip directories from filename prefixes. (PR #168355)

[Keith Smiley via llvm-commits]

keith wrote:

this doesn't fix the core bazel issue because this also affects the output path, so the assumption is that instead of outputting `foo/bar/R600GenRegisterInfo.inc` it outputs `R600GenRegisterInfo.inc` directly, assuming that it is run from the same directory where it should output the file

https://github.com/llvm/llvm-project/pull/168355