[llvm] [IR] Don't allow values of opaque type (PR #137625)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > This feels like it _should_ need a change in LangRef.
> 
> Done.

Thanks. But if I'm reading the markup right, the LangRef still has the "Opaque Structure Types" section as a subsection of the "First Class Types" section. (But this is really non-obvious to the reader anyway, so perhaps we should switch to a different way of documenting the different "kinds" of types.)

https://github.com/llvm/llvm-project/pull/137625