[clang][NFC] Refactor expected directives in C++ DRs 1-99 (llvm#73879)

This patch converts (almost) every expected directive in `test/CXX/drs/dr0xx.cpp` into either `@-1` form (when directive immediately follow the line diagnostic is pointing out to), or `@#<marker>` form (when directive is placed away from the line diagnostic is pointing out to). It also converts directive to match exactly one diagnostic, as opposed to matching multiple. Error messages are expanded to exactly match compiler output. `#if __cplusplus` guarding directives are replaced with respective prefixes (e.g. `since-cxx17`).

All aforementioned changes serve a purpose of making it easier to reconstruct expected compiler output, which should also make it a bit easier to grasp the gist of those already non-trivial tests due to their nature of testing corner cases of the language.