[clang] Cover CWG issues about `export template` (#94876)
This PR covers the following Core issues:
[CWG204](https://cplusplus.github.io/CWG/issues/204.html) "Exported
class templates"
[CWG323](https://cplusplus.github.io/CWG/issues/323.html) "Where must
`export` appear?"
[CWG335](https://cplusplus.github.io/CWG/issues/335.html) "Allowing
`export` on template members of nontemplate classes"
[CWG820](https://cplusplus.github.io/CWG/issues/820.html) "Deprecation
of `export`"
I believe the list above is entirety of Core issues that are dedicated
solely to `export template`.
I believe we have two main points of view here, which command what this
PR should do:
1. (easy) Removal of `export template` was done as a defect report in
CWG820, and the rest are effectively superseded by it, because we apply
defect reports retroactively.
2. (harder) Those Core issues are testable individually, so we should
test them for the behavior Core wanted at the time.
This PR implements the first option, making our C++ DR status page
greener.
I think I can be persuaded to go with the second option, if reviewers
have strong preference for it.
