gccrs: Add trait context to ContextualASTVisitor

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.

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