Properly handle attributes on statements

We now collect tokens for the underlying node wrapped by `StmtKind`
instead of storing tokens directly in `Stmt`.

`LazyTokenStream` now supports capturing a trailing semicolon after it
is initially constructed. This allows us to avoid refactoring statement
parsing to wrap the parsing of the semicolon in `parse_tokens`.

Attributes on item statements
(e.g. `fn foo() { #[bar] struct MyStruct; }`) are now treated as
item attributes, not statement attributes, which is consistent with how
we handle attributes on other kinds of statements. The feature-gating
code is adjusted so that proc-macro attributes are still allowed on item
statements on stable.

Two built-in macros (`#[global_allocator]` and `#[test]`) needed to be
adjusted to support being passed `Annotatable::Stmt`.

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -132,6 +132,7 @@ fn stmt_let_underscore(cx: &mut ExtCtxt<'_>, sp: Span, expr: P<ast::Expr>) -> as
         id: ast::DUMMY_NODE_ID,
         span: sp,
         attrs: ast::AttrVec::new(),
+        tokens: None,
     });
-    ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Local(local), span: sp, tokens: None }
+    ast::Stmt { id: ast::DUMMY_NODE_ID, kind: ast::StmtKind::Local(local), span: sp }
 }

compiler/rustc_builtin_macros/src/deriving/mod.rs

@@ -64,7 +64,6 @@ impl MultiItemModifier for BuiltinDerive {
                             id: ast::DUMMY_NODE_ID,
                             kind: ast::StmtKind::Item(a.expect_item()),
                             span,
-                            tokens: None,
                         })));
                     });
                 } else {