Fix closure with block body followed by method call (issue #4050)

[tao501]

Description

Fix for rustfmt issue #4050: "Rustfmt changes meaning of closure, also not idempotent"

Problem

When a closure with a block body is followed by a method call (e.g., || { ... }().to_string()), rustfmt incorrectly formats it as:

|| { { println!(); || 0 }().to_string(); }

This changes the program's semantics from ((|| {...}())..to_string()) to || {({...}()..to_string());}.

Root Cause

In src/chains.rs, the should_add_parens() function returns false for closures with block bodies, so the chain formatter doesn't add the necessary parentheses around the closure call before the method call.

Fix

Added ast::ExprKind::Block(..) => true to the closure match in should_add_parens(), ensuring that closures with block bodies get parenthesized when followed by method calls.

This produces the correct output:

(|| { println!(); || 0 }()).to_string()

Closes rust-lang#4050