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

[Jay Foad via llvm-commits]

jayfoad wrote:

> Is the intent mostly to find easily where different class or other defs are coming from (in other files)?

The use cases I had in mind are to do with debugging tablegen itself:

- If you want to reduce a test case (either manually or automatically) it's easier if you can start with a single file.
- If someone reports a tablegen bug that reproduces on an out-of-tree backend it's easier if they can give you a repro consisting of a single file.

But it seems like a generally useful thing that might find more use cases than I can imagine.

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