Implemented 90% functionality of new child_iterator for Stmt objects

that will (soon) allow iteration over the initializers in
declarations.  This new iterator mechanism is implemented by the
classes StmtIterator and ConstStmtIterator.

Patched a few files to use "operator++" instead of "operator+" on
child_iterators.

Friendship added in VarDecl to StmtIterator to allow returning a
reference to the initializer within the VarDecl.  We may not wish this
as a permanent solution.

llvm-svn: 43105