Turn -Wmicrosoft-enum-value off by default.

The warning is currently way too noisy to be useful. The plan is
to make it warn when an MS enum that's negative is compared to
something, but until that's done the warning shouldn't default
to on.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@302187 91177308-0d34-0410-b5e6-96231b3b80d8