[ASTMatchers] Add hasInClassInitializer traversal matcher for FieldDecl.

Summary:
I needed to know whether a FieldDecl had an in-class
initializer for D26453. I used a narrowing matcher there, but a
traversal matcher might be generally useful.

Reviewers: sbenza, bkramer, klimek, aaron.ballman

Subscribers: aaron.ballman, Prazek, cfe-commits

Differential Revision: https://reviews.llvm.org/D28034

llvm-svn: 290492