Ensure that CMake tries to find Python3 before processing

Summary:
It is possible that CMake tries to process debuginfo-tests before any
attempt to find Python3. Ensure that CMake attempts to find it before
complaining that it doesn't exist.

Reviewers: zturner, rnk, jmorse, chandlerc

Reviewed By: rnk

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79795