[5.10] Cherry-pick Fix divergent overwrite behavior of FileManager.copyItem(at:to:) on Linux and Windows

[gwynne]

This is a cherry-pick of #4821 to 5.10.

On Apple platforms, the FileManager APIs copyItem(atPath:toPath:) and copyItem(at:to:) refuse to overwrite the destination file (or link, or directory) if it already exists; this behavior is even explicitly documented. And indeed, any such attempt results in a fileWriteFileExists error.

However, on Linux and Windows, the destination is silently overwritten. This PR changes that behavior to match what's documented. On POSIX platforms, this is accomplished by adding the O_EXCL flag to the relevant open(2) call, which results in an EEXIST errno if the path already exists. For Windows, the bFailIfExists parameter of CopyFileW() is changed to true.

Fixes #3368

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform

[compnerd]

@swift-ci please test Windows platform