Fix a Cygwin build failure caused by enum values starting with '_', which is conflicted with some platform macros.
This solution only renames variables, no functional change.

NOTE: This is a candidate for the 3.4 branch.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195421 91177308-0d34-0410-b5e6-96231b3b80d8