llvm19: fix incremental builds

[brooks]

Summary

Detect and delete out of date clang-tblgen produced output. I found it
sufficent for compilation to delete only arm_*.inc files, but it seems
safer to regenerate all output and a few clang-tblgen pales in
comparison to the full build we're doing at this point.

[dim]

It's a big hammer, but better that than subtle errors...:)