TimeValue()::now().toEpochTime() is supposed to be the same as time(),
but it wasn't, because toEpoch subtracted PosixZeroTime, but now()
didn't add PosixZeroTime!
Add a unittest to check this works.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94178 91177308-0d34-0410-b5e6-96231b3b80d8
