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_interface/src/util.rs

@@ -810,7 +810,6 @@ impl<'a> MutVisitor for ReplaceBodyWithLoop<'a, '_> {
                 id: resolver.next_node_id(),
                 kind: ast::StmtKind::Expr(expr),
                 span: rustc_span::DUMMY_SP,
-                tokens: None,
             }
         }
 
@@ -827,7 +826,6 @@ impl<'a> MutVisitor for ReplaceBodyWithLoop<'a, '_> {
             id: self.resolver.next_node_id(),
             span: rustc_span::DUMMY_SP,
             kind: ast::StmtKind::Expr(loop_expr),
-            tokens: None,
         };
 
         if self.within_static_or_const {