Fix line-directive-tool not being able to invoke swiftc.exe on Windows

[hughbe]

• The first argument of command_args is the process to open.
• subprocess.Popen doesn't normalise arguments.
• This means that trying to open a non-normalised file (e.g. C:/swift/./bin/swiftc.exe) on Windows results in file/directory not found errors, as Popen delegates to the Win32 CreateProcess API.
• Unix systems handle non-normalised paths, so don't have this problem.
• Arguments passed to the process are normalised by the process, so are not affected.

[slavapestov]

@swift-ci Please smoke test

[compnerd]

This seems pretty reasonable, unfortunate, but reasonable.