Turn on clang by default on linux.

tools/clang/scripts/update.sh

Index: tools/clang/scripts/update.sh
diff --git a/tools/clang/scripts/update.sh b/tools/clang/scripts/update.sh
index 6eddf8fab0446358cf672d8e17fa4037f9fff309..85a33d6c51d9f531d8daf7fdf84a9e20856f8dbe 100755
--- a/tools/clang/scripts/update.sh
+++ b/tools/clang/scripts/update.sh
@@ -143,6 +143,12 @@ if [[ -n "$if_needed" ]]; then
     # clang previously downloaded, remove third_party/llvm-build to prevent
     # updating.
     true
+  elif [[ "${OS}" == "Linux" ]]; then
+    # Temporarily use clang on linux. Leave a stamp file behind, so that
+    # this script can remove clang again on machines where it was autoinstalled.
+    mkdir -p "${LLVM_BUILD_DIR}"
+    touch "${LLVM_BUILD_DIR}/autoinstall_stamp"
+    true
   else
     # clang wasn't needed, not doing anything.
     exit 0