c++: restore instantiate_decl assert

For PR61445 I removed this assert, but PR108242 demonstrated why it's still
useful; to avoid regressing the former testcase I check pattern_defined
in the assert.

This reverts r212524.

	PR c++/61445

gcc/cp/ChangeLog:

	* pt.cc (instantiate_decl): Assert !defer_ok for local
	class members.