[cmake] Move HandleOutOfTreeLLVM to common cmake utils

This is better than libunwind and libcxxabi fishing it out of libcxx's
module directory.

It is done in prepartion for a better version of D117537 which deduplicates
CMake logic instead of just renaming to avoid a name clash.

Reviewed By: phosek, #libunwind, #libc_abi, Ericson2314

Differential Revision: https://reviews.llvm.org/D117617