c++: explicit inst w/ similar constrained partial specs [PR104634]

c++: explicit inst w/ similar constrained partial specs [PR104634]

Here we neglect to emit the definitions of A<double>::f2 and A<double*>::f4
despite the explicit instantiations ultimately because TREE_PUBLIC isn't
set on the corresponding partial specializations, whose declarations are
created from maybe_new_partial_specialization which is responsible for
disambiguating them from the first and third partial specializations (which
have the same class-head but different constraints).  This makes grokfndecl
in turn clear TREE_PUBLIC for f2 and f4 as if they have internal linkage.

This patch fixes this by setting TREE_PUBLIC appropriately for such partial
specializations.

	PR c++/104634

gcc/cp/ChangeLog:

	* pt.cc (maybe_new_partial_specialization): Propagate TREE_PUBLIC
	to the newly created partial specialization.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/concepts-explicit-inst6.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>
(cherry picked from commit 47673571b28278d857371167f4b67a04a1b87b59)