Turn on clang by default on linux, 3rd try.

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 529873c3a772a62baee83ec1694fc718a4374a28..e86c288875f2f1f3d6de4cdb3ac8b3fe529e9f9c 100755
--- a/tools/clang/scripts/update.sh
+++ b/tools/clang/scripts/update.sh
@@ -137,13 +137,6 @@ while [[ $# > 0 ]]; do
   shift
 done
 
-# Remove clang on bots where it was autoinstalled in r281914.
-if [[ -f "${LLVM_BUILD_DIR}/autoinstall_stamp" ]]; then
-  echo Removing autoinstalled clang and clobbering
-  rm -rf "${LLVM_BUILD_DIR}"
-  rm -rf "${THIS_DIR}/../../../out"
-fi
-
 if [[ -n "$if_needed" ]]; then
   if [[ "${OS}" == "Darwin" ]]; then
     # clang is used on Mac.
@@ -155,6 +148,12 @@ if [[ -n "$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