gccrs: Change error location for LoopLabel

Loop label error reporting during ast validation was done at loop label
level. It lead to some innacurate error reporting in break loop labels
due to the way the label is built.

gcc/rust/ChangeLog:

	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Change
	reported error location to the lifetime location.

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