[ASTMatchers] Introduce Objective-C matchers `isClassMessage`,...

[ASTMatchers] Introduce Objective-C matchers `isClassMessage`, `isClassMethod`, and `isInstanceMethod`

Summary:
isClassMessage is an equivalent to isInstanceMessage for ObjCMessageExpr, but matches message expressions to classes.

isClassMethod and isInstanceMethod check whether a method declaration (or definition) is for a class method or instance method (respectively).

Contributed by @mywman!

Reviewers: benhamilton, klimek, mwyman

Reviewed By: benhamilton, mwyman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60920

llvm-svn: 358904