ADT: remove MinGW32 and Cygwin OSType enum

Remove the MinGW32 and Cygwin types from the OSType enumeration.  These values
are represented via environments of Windows.  It is a source of confusion and
needlessly clutters the code.  The cost of doing this is that we must sink the
check for them into the normalization code path along with the spelling.

Addresses PR20592.
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.