[llvm] [LangRef] asm clobber constrains: '~memory' allows reads and synchronization (PR #150191)

[Ralf Jung via llvm-commits]

RalfJung wrote:

@efriedma-quic @nikic what could unblock this PR? Is the question whether these are truly the semantics we want, or is it about what optimizations and backends are actually doing today?

Surely there needs to be *some* way to indicate that an asm block way contain fences or release/acquire accesses, and because this is easily overlooked it would be good to clarify this. :)

https://github.com/llvm/llvm-project/pull/150191