d: Use weak linkage for template symbols instead of gnu.linkonce (PR99914)

d: Use weak linkage for template symbols instead of gnu.linkonce (PR99914)

The default linkage of templates in the D language is now DECL_WEAK
instead of  DECL_ONE_ONLY, if supported.  This better matches the
expected override semantics of template symbols compiled to object code.

For example:

 module rt.config;
 template rt_flag()
 {
   pragma(mangle, "rt_flag") __gshared bool rt_flag = true;
 }

 module main;
 extern(C) __gshared bool rt_flag = false;

The above currently does not succeed in linking due to there being
multiple definitions of `rt_flag' in different sections that aren't
considered mergeable.

The compiler flag enabling toggling of this has been given a clearer
named `-fweak-templates', which distinguishes itself from G++ `-fweak',
which is intended only for testing.

gcc/d/ChangeLog:

	PR d/99914
	* d-lang.cc (d_init): Disable flag_weak_templates if no support for
	weak or one-only symbols.
	* d-tree.h (VAR_OR_FUNCTION_DECL_CHECK): New macro.
	(DECL_INSTANTIATED): New macro.
	(d_comdat_linkage): Remove declaration.
	(d_linkonce_linkage): Remove declaration.
	(set_linkage_for_decl): New declaration.
	* decl.cc (DeclVisitor::visit (StructDeclaration *)): Replace call to
	d_linkonce_linkage with setting DECL_INSTANTIATED.
	(DeclVisitor::visit (ClassDeclaration *)): Likewise.
	(DeclVisitor::visit (EnumDeclaration *)): Likewise.
	(DeclVisitor::visit (InterfaceDeclaration *)): Remove call to
	d_linkonce_linkage.
	(get_symbol_decl): Call set_linkage_for_decl instead of
	d_linkonce_linkage.
	(d_finish_decl): Call set_linkage_for_decl.
	(d_comdat_linkage): Made function static.  Only set DECL_COMDAT for
	DECL_INSTANTIATED decls.
	(d_linkonce_linkage): Remove function.
	(d_weak_linkage): New function.
	(set_linkage_for_decl): New function.
	* gdc.texi (Runtime Options): Rename -fno-weak to -fno-weak-templates,
	update documentation of option.
	* lang.opt (fweak): Rename option to ...
	(fweak-templates): ... this.  Update help string.
	* modules.cc (get_internal_fn): Add Prot parameter.  Set generated
	function flag.
	(build_internal_fn): Update call to get_internal_fn.
	(build_dso_cdtor_fn): Likewise.
	(register_moduleinfo): Call d_finish_decl on dso_slot_node and
	dso_initialized_node.
	* typeinfo.cc (TypeInfoVisitor::internal_reference): Call
	set_linkage_for_decl instead of d_comdat_linkage.
	(TypeInfoDeclVisitor::visit (TypeInfoDeclaration *)): Remove calls to
	d_linkonce_linkage and d_comdat_linkage.
	(get_cpp_typeinfo_decl): Likewise.

gcc/testsuite/ChangeLog:

	PR d/99914
	* gdc.dg/pr99914.d: New test.