[cmake] Emit an error for -DBUILD_SHARED_LIBS=ON on Windows

Summary:
The BUILD_SHARED_LIBS=ON build fails on Windows, so prevent users from
enabling it.

Reviewers: beanz, smeenai, compnerd, phosek

Reviewed By: beanz

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70161