Windows: update for LLVM API change

MinGW32 and Cygwin have been removed from the OSType enumeration.  They are
represented as environments, and the OS is Windows in both of those cases.
Simply remove the value from the switch.

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@215304 91177308-0d34-0410-b5e6-96231b3b80d8