[libcxx] Make return value of array<T, 0>.data() checked only for libc++

The section array.zero says: "The return value of data() is unspecified".
This patch marks all checks of the array<T, 0>.data() return value as
libc++ specific.

Reviewed as https://reviews.llvm.org/D55364.
Thanks to Andrey Maksimov for the patch.

llvm-svn: 348485