gccrs: Add a new visitor that gathers context information

gccrs: 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>