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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> We never implemented export template - and no one did, therefore we are not affected by changes to it (no one did except for EDG, i don't even know if they shipped it)

When I said "wording changes that do not affect implementations", I meant a hypothetical conforming implementation. In other words, `N/A` is for wording changes that can't be exhibited by writing programs and feeding them into the compiler.

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