c++/modules: explicit inst of constructor

c++/modules: explicit inst of constructor

The extern template __shared_ptr<filesystem::_Dir> in bits/fs_dir.h was
leading to an ICE in import_export_decl in 29_atomics/atomic_ref/address.cc
because we had the nonsensical combination of DECL_REALLY_EXTERN and
!DECL_INTERFACE_KNOWN.  This turned out to be because mark_decl_instantiated
was exiting early if TREE_ASM_WRITTEN since way back in the pre-cgraph days,
and expand_or_defer_fn_1 sets TREE_ASM_WRITTEN on maybe-in-charge ctors.
The mark_decl_instantiated code is long-obsolete, so let's just remove it.

gcc/cp/ChangeLog:

	* module.cc (trees_out::write_function_def): Check flag consistency.
	* pt.cc (mark_decl_instantiated): Ignore TREE_ASM_WRITTEN.