TSCLibc: add an implementation of `mkdtemp` for Windows
TSCLibc: add an implementation of `mkdtemp` for Windows

This adds an implementation of `mkdtemp` for Windows to accommodate
swift-package-manager which uses this rather than Foundation.

apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core,apple/swift-tools-support-core