[index/AST] Add references for ObjC getter=/setter= property attributes and related property getter/setter role fixes

This enhances the AST to keep track of locations of the names in those ObjC property attributes, and reports them for indexing.

Patch by Nathan Hawes!
https://reviews.llvm.org/D30907

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297972 91177308-0d34-0410-b5e6-96231b3b80d8