Don't interfere with the __builtin_foo namespace under MSVC

This patch follows up on feedback received in the review for
D32988. Specifically that libc++ should not mess with the
__builtin namespace, and that libc++ should use __popcnt to implement
__pop_count under MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302731 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed