c++, debug: Fix up locus of DW_TAG_imported_module [PR108716]

Before IMPORTED_DECL has been introduced in PR37410, we used to emit correct
DW_AT_decl_line on DW_TAG_imported_module on the testcase below, after that
change we haven't emitted it at all for a while and after some time
started emitting incorrect locus, in particular the location of } closing
the function.

The problem is that while we have correct EXPR_LOCATION on the USING_STMT,
when genericizing that USING_STMT into IMPORTED_DECL we don't copy the
location to DECL_SOURCE_LOCATION, so it gets whatever input_location happens
to be when it is created.

2023-03-02  Jakub Jelinek  <jakub@redhat.com>

	PR debug/108716
	* cp-gimplify.cc (cp_genericize_r) <case USING_STMT>: Set
	DECL_SOURCE_LOCATION on IMPORTED_DECL to expression location
	of USING_STMT or input_location.

	* g++.dg/debug/dwarf2/pr108716.C: New test.