[TestModuleCXX] Make this test Darwin-only.
[TestModuleCXX] Make this test Darwin-only.

Apparently this functionality is not expected to work on non-Darwin
systems. I should've checked the decorator on the original test.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@354487 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 4312bde31ffedcd39b47ecb4e3198c5822abca93)

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb