[CMake] Enforce the minimum CMake version to be at least 3.13.4

This commit changes the warning for CMake < 3.13.4 into a fatal error.
The intent is to revert and re-apply this simple commit until all build
bots are migrated to CMake >= 3.13.4.

This is part of the effort discussed on llvm-dev here:

	http://lists.llvm.org/pipermail/llvm-dev/2020-April/140578.html

Differential Revision: https://reviews.llvm.org/D78646