gccrs: Emit an error on unsafe modules

gccrs: Emit an error on unsafe modules

An error should be emitted on unsafe modules during the AST validation
pass as the syntax allows those even though they're not alowed later down
the line.

gcc/rust/ChangeLog:

	* ast/rust-item.h: Add safety getter to modules.
	* checks/errors/rust-ast-validation.cc (ASTValidation::visit): Check
	a module's safety and emit an error when meeting an unsafe module.
	* checks/errors/rust-ast-validation.h: Add function prototype.
	* parse/rust-parse-impl.h (Parser::parse_module): Move the module locus
	to the first token instead of the mod keyword.

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