[libc++] Use a function to set warning flags per target

[libc++] Use a function to set warning flags per target

This is part of a larger shift to move to per-target settings and
eradicate global variables from the CMake build. I'm starting small
with warnings only because those are easy to transition over and I
want to see how it pans out, but we can handle all flags like exceptions
and RTTI in the future.

llvm-svn: 373511