[llvm][Windows] Don't run socket tests on old versions of Windows

AF_UNIX sockets were added to Windows 10 build 17063 in 2017, older
versions of Windows will fail this test.

Also add a lit config so lit tests using sockets can do:
// REQUIRES: unix-sockets
(It would be cool if unit tests could use lit available_features)

Also fix llvm-config test that didn't fail when new libs are added.