Merge pull request #4170 from 9rnsr/refactor_ctfe

[Refactoring] Remove interpret member function from Expression and Statement
fix misleading deprecation messages
Merge pull request #4173 from WalterBright/errordep

fix misleading deprecation messages
Improve error messages in CTFE interpreter
[Refactoring] Make interpretFourPointerRelation static

Explicitly returning result from the non trivial helper function would be better than implicit result setting.
+stash for the code style changes