Revert "Bump required CMake version to 3.17"

Author: xymus

CMakeLists.txt

@@ -6,7 +6,7 @@
 # See http://swift.org/LICENSE.txt for license information
 # See http://swift.org/CONTRIBUTORS.txt for Swift project authors
 
-cmake_minimum_required(VERSION 3.17)
+cmake_minimum_required(VERSION 3.15.1)
 
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)