[libcxxabi][CMake] Drop unused HandleOutOfTreeLLVM include

This include doesn't seem to be needed for the standalone build (it's
not being used by libc++ build either), but introduces unnecessary
dependency because HandleOutOfTreeLLVM performs checks that require
a working C++ library. We shouldn't require a working C++ library to
build libc++abi or libc++ (it's what we're building after all).

Differential Revision: https://reviews.llvm.org/D58333

llvm-svn: 354284