Auto merge of #141864 - Berrysoft:cygwin-path, r=ChrisDenton

Handle win32 separator for cygwin paths

This PR handles a issue that cygwin actually supports Win32 path, so we need to handle the Win32 prefix and separaters.

r? `@mati865`

cc `@jeremyd2019`

~~Not sure if I should handle the prefix like the windows target... Cygwin *does* support win32 paths directly going through the APIs, but I think it's not the recommended way.~~

Here I just use `cygwin_conv_path` because it handles both cygwin and win32 paths correctly and convert them into absolute POSIX paths.

UPDATE: Windows path prefix is handled.

Author: bors