[CodeCompletion] Block property setters: Use dynamic priority heuristic

Now when completing blocks properties that return void the block call completion
result shows up before the setter, otherwise the setter completion shows up
before the block call completion. We normally want to use the result of the
block call, so one typically wouldn't call a block that returns a non-void type
in a standalone statement.

rdar://28846153

Differential Revision: https://reviews.llvm.org/D26034

llvm-svn: 291232