[llvm] [AST] Fix size merging for MustAlias sets (PR #73820)

[Nikita Popov via llvm-commits]

nikic wrote:

> I'm in favor of #65731 if it solves this issue as well.

I'd still like a non-intrusive fix for this that is suitable for backport in the meantime.

https://github.com/llvm/llvm-project/pull/73820