Improve handling of __FUNCTION__ and other predefined expression for Objective-C Blocks

Instead of always displaying the mangled name, try to do better
and get something closer to regular functions.

Recommit r287039 (that was reverted in r287039) with a tweak to
be more generic, and test fixes!

Differential Revision: https://reviews.llvm.org/D26522

llvm-svn: 287085