Introduce LocalSource into the AST.

This will be used to keep track of the origin of a local in the AST. In
particular, it will be used by `async fn` lowering for the locals in
`let <pat>: <ty> = __arg0;` statements.

src/libsyntax_ext/deriving/debug.rs

@@ -128,6 +128,7 @@ fn stmt_let_undescore(cx: &mut ExtCtxt<'_>, sp: Span, expr: P<ast::Expr>) -> ast
         id: ast::DUMMY_NODE_ID,
         span: sp,
         attrs: ThinVec::new(),
+        source: ast::LocalSource::Normal,
     });
     ast::Stmt {
         id: ast::DUMMY_NODE_ID,