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

[Keith Smiley via llvm-commits]

keith wrote:

the problem is that https://github.com/llvm/llvm-project/blob/dbd97c8d5521cfe80a6655b78e938cdb83742011/llvm/lib/TableGen/Main.cpp#L197-L200 happens after this removing of the directory portion of the path

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