Added back VisitDeclStmt() to the StmtDumper, essentially reverting r44920:

http://llvm.org/viewvc/llvm-project?rev=44920&view=rev

Putting VisitDeclStmt() was motivated because it called DumpDeclarator(),
which printed out a little bit more information than just using the
child_iterator interface to visit the subexpressions of DeclStmt. To avoid
printing the initializers twice, DumpSubTree() now specially checks for
DeclStmts; in such cases it calls VisitDeclStmt() without using the
child_iterators to visit the subexpressions.

llvm-svn: 44924