[nfc] Cleanup extension header generation

(cherry picked from commit f44a508)

Author: serge-sans-paille

llvm/cmake/modules/AddLLVM.cmake

@@ -937,17 +937,18 @@ function(process_llvm_pass_plugins)
           DESTINATION ${LLVM_INSTALL_PACKAGE_DIR}
           COMPONENT cmake-exports)
 
-      file(WRITE "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def.tmp" "//extension handlers\n")
+      set(ExtensionDef "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def")
+      file(WRITE "${ExtensionDef}.tmp" "//extension handlers\n")
       foreach(llvm_extension ${LLVM_STATIC_EXTENSIONS})
-        file(APPEND "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def.tmp" "HANDLE_EXTENSION(${llvm_extension})\n")
+          file(APPEND "${ExtensionDef}.tmp" "HANDLE_EXTENSION(${llvm_extension})\n")
       endforeach()
-      file(APPEND "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def.tmp" "#undef HANDLE_EXTENSION\n")
+      file(APPEND "${ExtensionDef}.tmp" "#undef HANDLE_EXTENSION\n")
 
       # only replace if there's an actual change
       execute_process(COMMAND ${CMAKE_COMMAND} -E copy_if_different
-        "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def.tmp"
-        "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def")
-      file(REMOVE "${LLVM_BINARY_DIR}/include/llvm/Support/Extension.def.tmp")
+          "${ExtensionDef}.tmp"
+          "${ExtensionDef}")
+      file(REMOVE "${ExtensionDef}.tmp")
   endif()
 endfunction()