PR c++/81866 - ICE with member template and default targ.

PR c++/81866 - ICE with member template and default targ.

This testcase manages to find a way to look up the partial instantiation of
B for the default argument of C before we've created the partial
instantiation of B as part of the normal instantiation of the members of A.
Which we can deal with, but we were getting confused because the partial
instantiation was stored with a RECORD_TYPE specialization rather than
TEMPLATE_DECL.

	* pt.c (tsubst_template_decl): Handle getting a type from
	retrieve_specialization.

From-SVN: r270138