Began implementing "child iterator" interface for Stmts and Exprs. Each

subclass of Stmt will implement child_begin() and child_end(), which will
be used to iterate over all the children (subexpressions/substatements) of
a Stmt object.  This will provide for easy traversal over the AST, which
is useful for a variety of purposes.

None of the interfaces to subclasses of Stmt will be changed (other than
adding the child_begin and child_end methods).

The only caveat is that the implementation of subclasses of Stmt will require
colocating all substatements (subexpressions) in an array.  This is because
we define child_iterator as Stmt**.  All accessor methods to subexpressions
will need to be altered to reflect this new implementation.

This patch includes the typedefs for child_iterator, as well the implementation
for child_begin/child_end for the primary expressions and some postfix
expressions.

llvm-svn: 41363