Don't set project version so that BASE_LLVM_VERSION isn't needed (#496)

It is tedious to update BASE_LLVM_VERSION for different opencl-clang
branches when llvm version changes.

In addition, project version doesn't bring obvious gain.

CMakeLists.txt

@@ -1,10 +1,5 @@
 cmake_minimum_required(VERSION 3.13.4)
 
-if(NOT DEFINED BASE_LLVM_VERSION)
-  set(BASE_LLVM_VERSION 18)
-endif(NOT DEFINED BASE_LLVM_VERSION)
-set(OPENCL_CLANG_VERSION ${BASE_LLVM_VERSION}.0)
-
 if(NOT DEFINED OPENCL_CLANG_BUILD_EXTERNAL)
   # check if we build inside llvm or not
   if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
@@ -14,8 +9,6 @@ endif(NOT DEFINED OPENCL_CLANG_BUILD_EXTERNAL)
 
 if(OPENCL_CLANG_BUILD_EXTERNAL)
   project(OPENCL_CLANG
-    VERSION
-    ${OPENCL_CLANG_VERSION}
     LANGUAGES
       CXX
       C