Revert "Reland "[CMake] Bumps minimum version to 3.20.0.""

This reverts commit 65429b9a.

Broke several projects, see https://reviews.llvm.org/D144509#4347562 onwards.

Also reverts follow-up commit "[OpenMP] Compile assembly files as ASM, not C"

This reverts commit 4072c8ae.

Also reverts fix attempt  "[cmake] Set CMP0091 to fix Windows builds after the cmake_minimum_required bump"

This reverts commit 7d47dac5.