[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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@373381 91177308-0d34-0410-b5e6-96231b3b80d8
7 files changed