[cmake] Prevent building with BUILD_SHARED_LIBS and LLVM_LINK_LLVM_DYLIB

Summary: This doesn't work, so error early if someone tries it.

Reviewers: beanz, smeenai, compnerd, phosek

Reviewed By: beanz

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70160