[CodeCompletion] Add a block property setter completion result

This commit changes code completion results for Objective-C block properties:
clang now suggests an additional completion result that displays the block
property together with '=' and the block literal placeholder for the appropriate
readwrite block properties.

This commit uses a simple heuristic to determine when it's appropriate to
suggest a setter completion for block properties: the additional block setter
completion is provided iff the member access that's being completed is a
standalone statement.

rdar://28481726

Differential Revision: https://reviews.llvm.org/D25520

llvm-svn: 284472