c++: inlining explicit instantiations [PR104539]

c++: inlining explicit instantiations [PR104539]

The PR10968 fix cleared DECL_COMDAT to force output of explicit
instantiations.  Then the PR59469 fix added a call to mark_needed, after
which we no longer need to clear DECL_COMDAT, and leaving it set allows us
to inline explicit instantiations without worrying about symbol
interposition.

I suppose there's an argument to be made that an explicit instantiation
declaration (extern template) should clear DECL_COMDAT, since that suggests
that there will be only a single instantiation somewhere that could be
subject to interposition, but that doesn't change the 'inline' semantics,
and it seems cleaner to treat template instantiations uniformly.

	PR c++/104539

gcc/cp/ChangeLog:

	* pt.cc (mark_decl_instantiated): Don't clear DECL_COMDAT.

gcc/testsuite/ChangeLog:

	* g++.dg/ipa/inline-4.C: New test.