Patch by GM: Turn off 'deprecated' warnings when building with MSVC, and add '-Werror=return-type' to catch funtions that aren't returning what they should.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@193088 91177308-0d34-0410-b5e6-96231b3b80d8