Make the Stmt::Profile method const, and the StmtProfile visitor

a ConstStmtVisitor. This also required adding some const iteration
support for designated initializers and making some of the getters on
the designators const.

It also made the formatting of StmtProfile.cpp rather awkward. I'm happy
to adjust any of the formatting if folks have suggestions. I've at least
fitted it all within 80 columns.

llvm-svn: 133152