[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. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@373511 91177308-0d34-0410-b5e6-96231b3b80d8 
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