[Sema][ObjC] Preserve syntactic sugar when removing

ARCReclaimReturnedObject cast.

This is a follow-up to r314370.

Rather than throwing away the enclosing parentheses, this commit walks
down the expression until an ARCReclaimReturnedObject cast is found and
removes just the cast, preserving the syntactic sugar expressions
(parens and casts) that were visited up to that point.

rdar://problem/34705720

Differential Revision: https://reviews.llvm.org/D38659

llvm-svn: 315261