[clang] [RFC] Initial implementation of P2719 (PR #113510)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> We have tracked two more issues to this commit. Both only manifest when using Clang header modules, which likely means that AST serialization is somehow incorrect after this patch. I'm reducing one of the test cases now, but it's taking a lot of time. In the meantime, please take a look at the AST serialization in case you can spot any issue there.

Will do, are you seeing a crash or codegen error? There were a bunch of serialization changes at the end so it's probable that I messed it up there (will be going through the serialization this evening)

https://github.com/llvm/llvm-project/pull/113510