Some construct are forbidden in trait context (eg. pub, async...) and
we'll need to reject those. To do so we need to identify a trait context.
gcc/rust/ChangeLog:
* ast/rust-ast-visitor.cc (ContextualASTVisitor::visit): Push the new
trait context when visiting a trait.
* ast/rust-ast-visitor.h: Add visit function prototype and TRAIT
context.