Add a new visitor that gathers context information

This visitor is intended to be used by other visitors that require
context at some point for a given item.

gcc/rust/ChangeLog:

	* ast/rust-ast-visitor.cc (ContextualASTVisitor::visit): Add multiple
	context saving calls.
	* ast/rust-ast-visitor.h (class DefaultASTVisitor): Make visit
	functions virtual.
	(class ContextualASTVisitor): Add a stack like container for the
	current context chain.

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

gcc/rust/ast/rust-ast-visitor.cc

@@ -1447,5 +1447,29 @@ DefaultASTVisitor::visit (AST::BareFunctionType &type)
     visit (type.get_return_type ());
 }
 
+void
+ContextualASTVisitor::visit (AST::Crate &crate)
+{
+  push_context (Context::CRATE);
+  DefaultASTVisitor::visit (crate);
+  pop_context ();
+}
+
+void
+ContextualASTVisitor::visit (AST::InherentImpl &impl)
+{
+  push_context (Context::INHERENT_IMPL);
+  DefaultASTVisitor::visit (impl);
+  pop_context ();
+}
+
+void
+ContextualASTVisitor::visit (AST::TraitImpl &impl)
+{
+  push_context (Context::TRAIT_IMPL);
+  DefaultASTVisitor::visit (impl);
+  pop_context ();
+}
+
 } // namespace AST
 } // namespace Rust