[llvm] [LangRef] Add a description of the semantics of call signatures. (PR #136189)

[Bruno Cardoso Lopes via llvm-commits]

https://github.com/bcardosolopes approved this pull request.

LGTM with minor nit.

I agree with @joker-eph this feels like a bug. However, this PR codifies existing behavior, which seems fair to me. Hope someone puts an RFC at some point though.

https://github.com/llvm/llvm-project/pull/136189