Summary:
When compiling with clang, also specify -Wno-nullability-completeness

We currently use this flag for internal Facebook builds using buck.  This
brings the CMake and buck compilation command lines closer in sync.  Even
though the folly code currently does not produce any nullability completeness
warnings, we are likely to break this in the future since we do not have
this warning enabled internally.

It also seems like `-Wnullability-completeness` results in code that cannot be
compiled successfully across various clang versions.  Recent versions of clang
complain about missing nullability specifiers on `std::unique_ptr` arguments,
while slightly older clang versions produce compilation errors if nullability
specifiers are listed on anything but raw pointers.

Reviewed By: igorsugak

Differential Revision: D6902930

fbshipit-source-id: 0b8cd5eca17cf8f9deda6b0c32f5838489542128