Fix a small difference in sema and codegen views of what needs to be output.

In the included testcase, soma thinks that we already have a definition after we
see the out of line decl. Codegen puts it in a deferred list, to be output if
a use is seen. This would break when we saw an explicit template instantiation
definition, since codegen would not be notified.

This patch adds a method to the consumer interface so that soma can notify
codegen that this decl is now required.

llvm-svn: 152024