Fix the attribute enable_if example in the docs to be 100% real-world, instead...

Fix the attribute enable_if example in the docs to be 100% real-world, instead of merely being close to the real world to mislead people. This now matches the test.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199298 91177308-0d34-0410-b5e6-96231b3b80d8