Emit error on identical use declarations

Emit error on identical use declarations

The compiler did not emit any warning when a same target was declared
from different sources.

gcc/rust/ChangeLog:

	* resolve/rust-toplevel-name-resolver-2.0.cc (TopLevel::handle_use_dec):
	Use the new dict to track down already resolved use declarations.
	* resolve/rust-toplevel-name-resolver-2.0.h: Add new dict to store
	previous use declarations.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>