Merge pull request #4163 from 9rnsr/refactor_ctfe

[Refactoring] A little code cleanup in CTFE interpreter
[Refactoring] Remove unused variable evaluatingArgs
[Refactoring] reduce code indent depth
Return NULL from WithStatement::interpret()
Improve error messages in CTFE interpreter
[Refactoring] Remove Statement::interpret()

Conflicts:
	src/interpret.c
[Refactoring] Remove Expression::interpret()

Conflicts:
	src/interpret.c
[Refactoring] Handle NULL Expression or Statement in interpret() call
[Refactoring] Add findGotoTarget to share common code
[Refactoring] Make interpretFourPointerRelation static

Explicitly returning result from the non trivial helper function would be better than implicit result setting.