pointerDifference() now returns UnionExp
Merge pull request #4162 from WalterBright/pointerdiff

CTFE: pointerDifference() now returns UnionExp
[Refactoring] fix code style in compilable/interpret3.d
[Refactoring] Add exceptionOrCant to make error handling shorter
[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.