[CMake] Detecting python modules should be cached

Summary: This requres exec-ing python, which in a trace I ran of the CMake re-configure time took ~2% of the reconfigure time.

Reviewers: phosek, smeenai, compnerd

Subscribers: mgorny, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61402

llvm-svn: 360196