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

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cor3ntin 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)

It is perfectly reasonable not to have tests whatsoever for that either (although I find the pre-c++20 diagnostic funny, we would be better of saying that modules are a c++20 feature). 

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