[cmake] Bump OpenCilk version to 4.0.0.

Author: neboat

cmake/Modules/LLVMVersion.cmake

@@ -14,7 +14,7 @@ if(NOT DEFINED LLVM_VERSION_SUFFIX)
 endif()
 
 if(NOT DEFINED OPENCILK_VERSION_MAJOR)
-  set(OPENCILK_VERSION_MAJOR 3)
+  set(OPENCILK_VERSION_MAJOR 4)
 endif()
 if(NOT DEFINED OPENCILK_VERSION_MINOR)
   set(OPENCILK_VERSION_MINOR 0)