Printing backtrace will leak PDB file handles

backtrace-rs uses StackWalkEx on Windows, which calls SymLoadModuleEx in turn. The symbol modules are never unloaded afterward, keeping the PDB files open until the process exits.

Normally this wouldn't be an issue, since most programs exit on panic anyway. But it's causing rust-lang/rust-analyzer#9932, where when a proc macro panics in rust-analyzer, it locks the PDB file and making subsequent build attempts fail:

note: LINK : fatal error LNK1201: error writing to program database 'target\debug\deps***_derive-fd4461955b1f812b.pdb';
check for insufficient disk space, invalid path, or insufficient privilege

Steps to reproduce

// Run with `RUST_BACKTRACE=1`
fn main() {
    let _ = std::panic::catch_unwind(|| panic!());
    loop { std::thread::park(); }
}

Start by tracing the Windows backtrace path that calls StackWalkEx and SymLoadModuleEx, then inspect how symbol modules are managed after a backtrace is printed. Use the provided catch_unwind reproduction with RUST_BACKTRACE=1 and verify that the PDB file is no longer held open while the process remains running.