[libc++][test] Use <version> to get config on MSVC

...instead of `<ciso646>`.

Also includes some NFC comment changes.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@374854 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed