Recommit r312127: [refactor] AST selection tree should contain syntactic

form of PseudoObjectExpr

The new commit adjusts unittest test code compilation options so that the
Objective-C code in the unittest can be parsed on non-macOS platforms.

Original message:

The AST selection finder now constructs a selection tree that contains only the
syntactic form of PseudoObjectExpr. This form of selection tree is more
meaningful when doing downstream analysis as we're interested in the syntactic
features of the AST and the correct lexical parent relation.

llvm-svn: 312132