[CMake] Support overriding CLANG_VERSION_*

As with how we handle LLVM_VERSION_* variables we should support
overriding clang version variables.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@282003 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -261,10 +261,16 @@ if( NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR )
   endif()
 endif()
 
-# Compute the Clang version from the LLVM version.
-set(CLANG_VERSION_MAJOR ${LLVM_VERSION_MAJOR})
-set(CLANG_VERSION_MINOR ${LLVM_VERSION_MINOR})
-set(CLANG_VERSION_PATCHLEVEL ${LLVM_VERSION_PATCH})
+# If CLANG_VERSION_* is specified, use it, if not use LLVM_VERSION_*.
+if(NOT DEFINED CLANG_VERSION_MAJOR)
+  set(CLANG_VERSION_MAJOR ${LLVM_VERSION_MAJOR})
+endif()
+if(NOT DEFINED CLANG_VERSION_MINOR)
+  set(CLANG_VERSION_MINOR ${LLVM_VERSION_MINOR})
+endif()
+if(NOT DEFINED CLANG_VERSION_PATCHLEVEL)
+  set(CLANG_VERSION_PATCHLEVEL ${LLVM_VERSION_PATCH})
+endif()
 # Unlike PACKAGE_VERSION, CLANG_VERSION does not include LLVM_VERSION_SUFFIX.
 set(CLANG_VERSION "${CLANG_VERSION_MAJOR}.${CLANG_VERSION_MINOR}.${CLANG_VERSION_PATCHLEVEL}")
 message(STATUS "Clang version: ${CLANG_VERSION}")