[clang] [clang] Add tests for DRs about complete-class context (PR #77444)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

This PR has been migrated from https://reviews.llvm.org/D148433.
I made non-functional changes to `expected` directives to match the rest of the DR test suite.
I also moved CWG2335 test in a dedicated file in order to avoid issues with test isolation. Clang can stop instantiating templates if an error occurred earlier in a TU, which leads to less diagnostics emitted. Test for CWG2335 runs into this issue.

https://github.com/llvm/llvm-project/pull/77444