[ASTDump] Add utility for dumping a label with child nodes

Summary:
Use it to add optional label nodes to Stmt dumps.  This preserves
behavior of InitExprList dump:

// CHECK-NEXT: `-InitListExpr {{.+}} <col:13, col:15> 'U [3]'
// CHECK-NEXT:   |-array_filler: InitListExpr {{.+}} <col:15> 'U' field Field {{.+}} 'i' 'int'
// CHECK-NEXT:   `-InitListExpr {{.+}} <col:14> 'U' field Field {{.+}} 'i' 'int'
// CHECK-NEXT:     `-IntegerLiteral {{.+}} <col:14> 'int' 1

Reviewers: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D55488

llvm-svn: 350957