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.