[clang] [clang][Modules] Complete the implementation of P2615: Meaningful exports (PR #194201)

[Victor Chernyakin via cfe-commits]

localspook wrote:

> As P2615R1 is the paper resolving CWG2443, I don't think it's possible to implement the paper without implementing CWG2443.

> I agree that this PR seems to implement CWG2443. If the author agrees, you should add (but not move) relevant tests to C++ DR tests and claim conformance with CWG2443

Agreed with both of those. I've added tests for CWG2443 by just copy-pasting all the tests involving `export` from the P2615 file. This introduces a fair bit of duplication into the tests though; are we fine with that? Or should I trim down the tests in the CWG2443 file?

> Please also include test cases with `export` blocks, which I expect to behave differently than `export` specifier on declarations.

These should be there already.

https://github.com/llvm/llvm-project/pull/194201