Suppress -Wdelete-non-virtual-dtor warnings about classes defined in system headers.

r312167 made it so that we emit Wdelete-non-virtual-dtor from delete statements
that are in system headers (e.g. std::unique_ptr). That works great on Linux
and macOS, but on Windows there are non-final classes that are defined in
system headers that have virtual methods but non-virtual destructors and yet
get deleted through a base class pointer (e.g. ATL::CAccessToken::CRevert). So
paddle back a bit and don't emit the warning if it's about a class defined in a
system header.

https://reviews.llvm.org/D37324

llvm-svn: 312216