Rollup merge of #34436 - jseyfried:no_block_expr, r=eddyb

To allow these braced macro invocation, this PR removes the optional expression from `ast::Block` and instead uses a `StmtKind::Expr` at the end of the statement list.

Currently, braced macro invocations in blocks can expand into statements (and items) except when they are last in a block, in which case they can only expand into expressions.

For example,
```rust
macro_rules! make_stmt {
    () => { let x = 0; }
}

fn f() {
    make_stmt! {} //< This is OK...
    let x = 0; //< ... unless this line is commented out.
}
```

Fixes #34418.

src/libsyntax_ext/deriving/mod.rs

@@ -298,8 +298,7 @@ fn call_intrinsic(cx: &ExtCtxt,
     let call = cx.expr_call_global(span, path, args);
 
     cx.expr_block(P(ast::Block {
-        stmts: vec![],
-        expr: Some(call),
+        stmts: vec![cx.stmt_expr(call)],
         id: ast::DUMMY_NODE_ID,
         rules: ast::BlockCheckMode::Unsafe(ast::CompilerGenerated),
         span: span }))