WG21-P3612R1 Harmonize Proxy-Reference Operations

No feature-test macro is mentioned.

This resolves:
LWG-3638 vector<bool>::swap(reference, reference) is useless
LWG-4187 bitset::reference should be const-assignable

This deprecates vector<bool>::swap(reference, reference), but as Wakely argued, it "is not useful. Nobody calls swap like that." As this simply resolves LWG issues and doesn't take any top-level names, I believe we should implement this unconditionally, and I'm willing to review a PR for it before opening up the general C++26 floodgates.