Only eat semicolons for statements that need them

When parsing a statement (e.g. inside a function body),
we now consider `struct Foo {};` and `$stmt;` to each consist
of two statements: `struct Foo {}` and `;`, and `$stmt` and `;`.

As a result, an attribute macro invoke as
`fn foo() { #[attr] struct Bar{}; }` will see `struct Bar{}` as its
input. Additionally, the 'unused semicolon' lint now fires in more
places.

compiler/rustc_parse/src/parser/stmt.rs

@@ -473,8 +473,7 @@ impl<'a> Parser<'a> {
                 // so capture it
                 add_semi_token(local.tokens.as_mut());
             }
-            StmtKind::Empty => eat_semi = false,
-            _ => {}
+            StmtKind::Empty | StmtKind::Item(_) | StmtKind::Semi(_) => eat_semi = false,
         }
 
         if eat_semi && self.eat(&token::Semi) {