CTFE: Interpret delegate expressions properly

&expr.func needs to work for arbitrary expr, not just VarExp.
Tighten the CTFE sanity checks.
Fix issue 10452 cannot compare delegates in CTFE

Support == and 'is' for delegates.
Merge pull request #2360 from donc/delegatecmp10452ctfe

10452  CTFE: Cannot compare delegates with == or 'is'