Rename StmtDumper::DumpSubTree() to dumpStmt()

Summary:
Also rename DumpDeclarator() to dumpDecl(). Once Decl dumping is added, these will be the two main methods of the class, so this is just for consistency in naming.

There was a DumpStmt() method already, but there was no point in having it, so I have merged it into VisitStmt(). Similarly, DumpExpr() is merged into VisitExpr().

Reviewers: alexfh

Reviewed By: alexfh

CC: cfe-commits, alexfh

Differential Revision: http://llvm-reviews.chandlerc.com/D156

llvm-svn: 169865