CodeGen: remove MinGW and Cygwin legacy entries

These are now treated as environments.  Remove references to these enumeration
values in order to clean up the unused enumeration entries in LLVM.  The target
normalisation prior to tool invocation should ensure that the old values
continue to function properly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209068 91177308-0d34-0410-b5e6-96231b3b80d8