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_parse/src/parser/nonterminal.rs

@@ -117,8 +117,8 @@ impl<'a> Parser<'a> {
                 let (stmt, tokens) = self.collect_tokens(|this| this.parse_stmt())?;
                 match stmt {
                     Some(mut s) => {
-                        if s.tokens.is_none() {
-                            s.tokens = tokens;
+                        if s.tokens().is_none() {
+                            s.set_tokens(tokens);
                         }
                         token::NtStmt(s)
                     }