c++: consolidate location printing in error.cc [PR116253]

Consolidate the location-printing logic in cp/error.cc, as preliminary
work towards supporting nested diagnostics (PR other/116253).

gcc/cp/ChangeLog:
	PR other/116253
	* error.cc (print_location): Move to earlier in the file.
	(print_instantiation_partial_context_line): Replace
	location-printing logic with a call to print_location.
	(print_instantiation_partial_context): Likewise, splitting up
	pp_verbatim calls.
	(maybe_print_constexpr_context): Likewise.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>