[lit] Support for Windows 10 Linux subsystem

Hello all,

we have recently experienced a problem on one of our win10 builders: lit
:: shtest-format test failed because system %PATH% variable contained
C:\windows\system32 folder before MSYS folder and therefore lit chose
windows builtin bash instead of msys. Builtin bash requires the
following format of arguments: bash /mnt/c/path/to/my/script but msys
bash uses "bash C:\path\to\my\script" and the latter is provided by lit
when invoking external shell. The problem is not only in bash in other
utils too (haven't conducted deep research yet).

For now I just raised priority of msys path on that builder, but the
question is: should we add support for Windows 10+ Linux subsystem? I
think lit should detect it and take this fact into account when running