This change fixes the symbol merging in get_symbol_decl to also consider
prototypes.  This allows the ability to set user defined attributes on
the prototype of a function, which then get applied to the definition,
if found later in the compilation.

The lowering of UDAs to GCC attributes has been commonized into a single
function called apply_user_attributes.

gcc/d/ChangeLog:

	PR d/90136
	* d-attribs.cc: Include dmd/attrib.h.
	(build_attributes): Redeclare as static.
	(apply_user_attributes): New function.
	* d-tree.h (class UserAttributeDeclaration): Remove.
	(build_attributes): Remove.
	(apply_user_attributes): Declare.
	(finish_aggregate_type): Remove attrs argument.
	* decl.cc (get_symbol_decl): Merge declaration prototypes with
	definitions.  Use apply_user_attributes.
	* modules.cc (layout_moduleinfo_fields): Remove last argument to
	finish_aggregate_type.
	* typeinfo.cc (layout_classinfo_interfaces): Likewise.
	* types.cc (layout_aggregate_members): Likewise.
	(finish_aggregate_type): Remove attrs argument.
	(TypeVisitor::visit (TypeEnum *)): Use apply_user_attributes.
	(TypeVisitor::visit (TypeStruct *)): Remove last argument to
	finish_aggregate_type.  Use apply_user_attributes.
	(TypeVisitor::visit (TypeClass *)): Likewise.

gcc/testsuite/ChangeLog:

	PR d/90136
	* gdc.dg/pr90136a.d: New test.
	* gdc.dg/pr90136b.d: New test.
	* gdc.dg/pr90136c.d: New test.