[AST] Add a method to get a call type from an ObjCMessageExpr
Due to references, expression type does not always correspond to an
expected method return type (e.g. for a method returning int & the
expression type of the call would still be int).
We have a helper method for getting the expected type on CallExpr, but
not on ObjCMessageExpr.
Differential Revision: https://reviews.llvm.org/D57204