c++: friend vs inherited guide confusion [PR117855]

c++: friend vs inherited guide confusion [PR117855]

We recently started using the lang_decl_fn::context field to track
inheritedness of a deduction guide (for C++23 inherited CTAD).  This
new overloading of the field accidentally made DECL_FRIEND_CONTEXT
return non-NULL for inherited guides, which breaks the below testcase
during overload resolution with an inherited guide.

This patch fixes this by refining DECL_FRIEND_CONTEXT appropriately.

	PR c++/117855

gcc/cp/ChangeLog:

	* cp-tree.h (DECL_FRIEND_CONTEXT): Exclude deduction guides.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/class-deduction-inherited7.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>