[llvm] [TableGen] Implement a preprocessor (PR #120113)

[Jay Foad via llvm-commits]

jayfoad wrote:

This patch is a bit of an RFC to see if there is any interest in having a preprocessor. I found it useful when I was trying to narrow down a tablegen crash, and I had no idea which part of the (AMDGPU) backend tablegen code was causing it. Preprocessing to a single file made it much easier to reduce the input.

https://github.com/llvm/llvm-project/pull/120113