PR c++/87748 - substitution failure error with decltype.

PR c++/87748 - substitution failure error with decltype.

This issue is similar to PR 87480; in both cases we were doing non-dependent
substitution with processing_template_decl set, leading to member access
expressions seeming still instantiation-dependent, and therefore decltype
not being simplified to its actual type.  And as in that PR, the fix is to
clear processing_template_decl while substituting a default template
argument.

	* pt.c (most_specialized_partial_spec): Clear
	processing_template_decl.

From-SVN: r269921