Bug 12905 – [AA] `opCmp` in any struct allow other struct usage in associative array

This code compiles but shouldn't: --- struct T { int opCmp(); } struct S {} int[S] aa; // Compiles iff `T` with `opCmp` exists --- Currently the compiler tries to enforce `opCmp` existence on AA keys so this is accepts-invalid bug. Also there is Issue 12255 complaining about `opCmp` requirement, but anyway this code must either compile or not without depending on `T` existence. IMO, this bug causes huge confusion in AAs and should be fixed ASAP.

In git head, int[S] is valid AA type, and equality and hashing of key object is done by the bit representation.