Handle multibyte paths on Windows

[lxbndr]

DispatchIO is used in FoundationNetworking.URLSession for a file reading. The path is taken from URL.withUnsafeFileSystemRepresentation, which looks as expected behavior to me. It is multibyte string in fact, and on the Dispatch side we have to handle this accordingly.

We noted this as crashes/hangs in our product, happening when we tried to POST files with non-ascii characters in the path. We are running with this patch without any issues for a few months already, so it is kind of tested. We're running on 5.10.1 release branch, but as long as withUnsafeFileSystemRepresentation is not changed, everything should be the same.

[lxbndr]

cc @compnerd

[lxbndr]

Update:

• Renamed wide_path to wszPath to match Windows code style
• Extracted code related to MultiByteToWideChar into a helper. Basically followed this code from swiftlang/swift.
• Added error handling. That was missing in original PR, and errors were crashing the app/tests. Now we gracefully call cleanup handler in case of error.

@compnerd please take a look

[lxbndr]

Reworked version

• some type/function aliasing for Win/Non-Win platforms
• path in dispatch_io_path_data_s is now WCHAR[] on Windows
• one single conversion from utf-8 to mbcs
• general flow is same, no new edge cases
• most significant changes are in dispatch_io_create_with_path part related to path data initialization

cc @compnerd

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform

[compnerd]

@swift-ci please test

[compnerd]

@swift-ci please test Windows platform