[ASTMatchers] Introduce a matcher for `ObjCIvarExpr`, support getting it's declaration

ObjCIvarExpr is *not* a subclass of MemberExpr, and a separate matcher
is required to support it.
Adding a hasDeclaration support as well, as it's not very useful without
it.

Differential Revision: https://reviews.llvm.org/D49701

llvm-svn: 338140