Make lit :: shtest-format.py supported on Windows again

It was marked as unsupported on Windows in r311230 because on some Win10 
machines it failed or caused hang. The problem was that on these machines
system bash (C:\Windows\System32\bash.exe) was used which requires paths to be
passed like '/mnt/c/path/to/my/script' instead of 'C:\path\to\my\script'.

TODO: we should make lit detect if system bash is used instead of msys and set
appropriate path format.

llvm-svn: 311558