[clang] [Feat] Allow Finding across only parts of an AST. (PR #127423)

[Hans Wennborg via cfe-commits]

zmodem wrote:

> This is relevant for clang modules, as they are imported into the AST, but are actually part of a different TU.

I'm not a modules or matcher expert, but this doesn't match my (perhaps uninformed) understanding. I thought that when we import a module, it *is* part of our TU, similarly to when we include a header.

I thought what we wanted to do was avoid walking the parts of the AST that come from a module but aren't used in the TU, and therefore haven't been deserialized.

https://github.com/llvm/llvm-project/pull/127423