NFC: Move VisitExpr code to dumpStmt

Summary:
The call is duplicated in the handlers of all Expr subclasses.

This change makes it easy to split statement handling out to
TextNodeDumper.

Reviewers: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55339

llvm-svn: 348546