build-llvm: Unset CLEAN in build-llvm for --macos-native-tools

In this case, we want to incrementally build the remaining tools,
not do a full build. In case someone runs build-all.sh with CLEAN
set, the --macos-native-tools invocation of build-llvm.sh shouldn't
wipe the build directory.

build-llvm.sh

@@ -71,6 +71,7 @@ while [ $# -gt 0 ]; do
         ;;
     --macos-native-tools)
         MACOS_NATIVE_TOOLS=1
+        unset CLEAN
         ;;
     --instrumented|--instrumented=*)
         INSTRUMENTED="${1#--instrumented}"