Remove a long-standing __has_include hack.

This was put in to get libc++ building without libcxxabi. We now have
macros that show that we are building against libcxxabi so use that
instead. This guards against existing but broken cxxabi.h headers on the
system.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@250507 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed