Codechange: move CheckAtomic.cmake to its own folder together with the license text

CMakeLists.txt

@@ -343,7 +343,7 @@ if(NOT OPTION_DEDICATED)
     endif()
 endif()
 
-include(CheckAtomic)
+include(3rdparty/llvm/CheckAtomic)
 
 if(APPLE)
     link_package(Iconv TARGET Iconv::Iconv)

README.md

@@ -212,8 +212,8 @@ See `src/3rdparty/monocypher/LICENSE.md` for the complete license text.
 The OpenTTD Social Integration API in `src/3rdparty/openttd_social_integration_api` is licensed under the MIT license.
 See `src/3rdparty/openttd_social_integration_api/LICENSE` for the complete license text.
 
-The atomic datatype support detection in `cmake/CheckAtomic.cmake` is licensed under the Apache 2.0 license.
-See `src/3rdparty/llvm/14.0.0-LICENSE.TXT` for the complete license text.
+The atomic datatype support detection in `cmake/3rdparty/llvm/CheckAtomic.cmake` is licensed under the Apache 2.0 license.
+See `cmake/3rdparty/llvm/LICENSE.txt` for the complete license text.
 
 ## 4.0 Credits
 

cmake/3rdparty/llvm/CheckAtomic.cmake

@@ -1,3 +1,11 @@
+#
+# This was originally part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+# See (https://llvm.org/)LICENSE.txt for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+#
+# Modifications have been made to suit building OpenTTD.
+#
+
 # atomic builtins are required for threading support.
 
 INCLUDE(CheckCXXSourceCompiles)