Make ninja smart console builds more pretty

Summary: CMake's `find_package` outputs to the console on success, which confuses the smart console mode of the `ninja` build system. Let's quiet the success message and manually warn instead.

Reviewers: tstellar, phosek, mehdi_amini

Reviewed By: mehdi_amini

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D82276