revert:

commit 1d3f61f8a799489f7f4d81e46d9e6b31a954ea4e
Author: Louis Dionne <ldionne@apple.com>
Date:   Wed Apr 22 11:16:27 2020 -0400

    [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

--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -3,9 +3,11 @@
 cmake_minimum_required(VERSION 3.4.3)
 
 if ("${CMAKE_VERSION}" VERSION_LESS "3.13.4")
-  message(FATAL_ERROR
-    "Your CMake version is ${CMAKE_VERSION}. The minimum version of CMake "
-    "required to build LLVM is now 3.13.4.")
+  message(WARNING
+    "Your CMake version is ${CMAKE_VERSION}. Starting with LLVM 12.0.0, the "
+    "minimum version of CMake required to build LLVM will become 3.13.4, and "
+    "using an older CMake will become an error. Please upgrade your CMake to "
+    "at least 3.13.4 now to avoid issues in the future!")
 endif()
 
 if(POLICY CMP0068)