[clang] [clang] Improved diagnostics for explicit specialization/instantiation of closure type members (PR #192843)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> > @StefanPaulet Do you need us to merge this for you? Thanks!
> 
> We should not be merging with "changes requested" reviews. @ojhunt can you review this again (and hopefully approve)?

will do, one moment

https://github.com/llvm/llvm-project/pull/192843