llvm: cmake: set CMP0114 to OLD to avoid warning

Building with cmake 3.23.2+ produces warnings when using
LLVM_ENABLE_RUNTIMES. Set this policy to the existing behavior for now
to silence the warnings.

Fixes: https://github.com/llvm/llvm-project/issues/50122
Fixes: https://github.com/llvm/llvm-project/issues/54944

Reviewed By: phosek, penzn

Differential Revision: https://reviews.llvm.org/D126923