DWARF: materialize subprogram renamings in Ada as imported declarations

DWARF: materialize subprogram renamings in Ada as imported declarations

... so that debugger users can reference them instead of the renamed
subprograms.  This is in accordance with the DWARF specification: the
section 3.2.3 (Imported (or Renamed) Declaration Entities) allows
DW_TAG_imported_declaration DIEs to be used "as a general means to
rename or provide an alias for an entity regardless of the context in
which the importing declaration or the imported entity occur." (wording
from the DWARFv3 and DWARFv4 specifications, allowed in DWARFv2).

gcc/ada/ChangeLog:

	* gcc-interface/utils.c (gnat_write_global_declarations): Output
	debugging information for top-level imported declarations.

gcc/ChangeLog:

	* dwarf2out.c (gen_decl_die): Generate DW_TAG_imported_* instead
	of DW_TAG_namespace for IMPORTED_DECL declarations.  Call
	dwarf2out_imported_module_or_decl_1 for all DWARF versions as
	this function already takes care of checking what it can emit.

Bootstrapped, regtested and checked with GDB's testsuite on
x86_64-linux.

From-SVN: r230821