[libc++abi] Remove uses of C++ headers when possible

This reduces the (circular) dependency of libc++abi on a C++ standard
library. Outside of the demangler which uses fancier C++ features, the
only C++ headers now required by libc++abi are pretty much <new> and
<exception>, and that's because libc++abi defines some types that are
declared in those headers.

llvm-svn: 373381