[libc++] Enable <chrono> ""d and ""y literals for AppleClang 10 and up

AppleClang supports those literals starting in version 10.0.1.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@368882 91177308-0d34-0410-b5e6-96231b3b80d8