Upstream (impressively quickly) fixed the missing intrinsic. Switch Windows
clang back to building the same code as MSVC. Also include the intrin.h header
rather than forward-declare the intrinsic. clang only works if the header is
explicitly included. Chromium forcibly includes it to work around these kinds
of issues, but we shouldn't rely on it.
BUG=crbug.com/438382
Change-Id: I0ff6d48e1a3aa455cff99f8dc4c407e88b84d446
Reviewed-on: https://boringssl-review.googlesource.com/2461
Reviewed-by: Adam Langley <agl@google.com>