[clang] [clang] Cover CWG issues about `export template` (PR #94876)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

Thank you for chiming in!

> It might be fine to give a different color to superseded, but I would guess we'd want that to be a lighter form version of whatever the superseding issue is colored. e.g., given that conforming = green and non-conforming = red, if we have a superseded issue we conform to, it would be light green, and if we didn't conform it would be pink. WDYT?

I'm thinking of reducing opacity of the color instead of picking exact shades, but yeah, we can definitely do something of this sort.

https://github.com/llvm/llvm-project/pull/94876