d: Refactor DECL_ARGUMENT and DECL_RESULT generation to own function

d: Refactor DECL_ARGUMENT and DECL_RESULT generation to own function

When looking into PR109108, the reason why things go awry is because
of the logic around functions with thunks - they have their definitions
generated even when they are external.  This subsequently then relied on
the detection of whether a function receiving codegen really is extern
or not, and this check ultimately prunes too much.

This is a first step to both removing the call to `build_decl_tree' from
`make_thunk' and the pruning of symbols within the `build_decl_tree'
visitor method for functions.  Move the generation of DECL_ARGUMENT and
DECL_RESULT out of `build_decl_tree' and into their own functions.

gcc/d/ChangeLog:

	* decl.cc (get_fndecl_result): New function.
	(get_fndecl_arguments): New function.
	(DeclVisitor::visit (FuncDeclaration *)): Adjust to call
	get_fndecl_arguments.
	(make_thunk): Adjust to call get_fndecl_arguments and
	get_fndecl_result.
	(start_function): Adjust to call get_fndecl_result.