[CMake][NewPM] Move ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER into llvm/

This allows us to use its value everywhere, rather than just clang. Some
other places, like opt and lld, will use its value soon.

The #define for it is now in llvm-config.h.

Reviewed By: rnk, hans

Differential Revision: https://reviews.llvm.org/D92072