Atomic file writing for WASI

[MaxDesiatov]

Mirrored from swiftwasm/swift#5594

Description

Currently, Swift doesn't support the following code when using Wasm Swift SDK.

import Foundation
Data().write(to: URL(filePath: "hoge"), options: .atomic)

It's because the option .atomic requires creating a temporary file, and swift-foundation doesn't support it if os(WASI).

Ideas

One idea: swiftwasm/swift#5593 (comment)

How about allowing atomic file writing when TMPDIR environment variable is explicitly defined?