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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> @efriedma-quic merging this within 24h while a discussion is on-going seems like not the usual norm in the community, and clearly not what's encoding in our developer guide right now.

As far I can tell, there weren't any objections to the patch itself.  The discussion is centered around concerns that LLVM should not work this way... but LLVM works the way it does no matter what LangRef says.  If there's some specific concern, I'm happy to revise the text.

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