Ensure that ENABLE_SHARED is substituted for use in Windows builds

We (perhaps over-cautiously) disable the new plugin tests on static Windows
builds right now, matching what LLVM core does. This change was needed for the
lit check to work.

Thanks to Warren Hunt for spotting this.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198782 91177308-0d34-0410-b5e6-96231b3b80d8