c++: Propagate assembler name from local-externs [PR 99508]

c++: Propagate assembler name from local-externs [PR 99508]

This is another place where our one-true-decl representation breaks
down.  The fix here propagates the assembly name to the ns-scope
alias.  that fixes the reported problem but changes the behaviour when
the user has explicitly declared the entity in its namespace.
However, we didn't handle that case 'correctly' anyway before.
Previously we'd also ignore the explicitly specified assembler name,
now we propagate it.  It's not clear to me what the desired semantics
would be in decorating just one of the local extern declarations this
way.  I don't think we can really do better without propagating this
aliasing property into the middle end (which is also needed for some
constexpr handling, see PR97306).  I tried that before and it turned
into a rat-hole.

	PR c++/99508
	gcc/cp/
	* decl.c (make_rtl_for_nonlocal_decl): Propagate local-extern's
	assembler name to the ns alias.
	gcc/testsuite/
	* g++.dg/ext/pr99508.C: New.