Clang update script: disable --with-android with LLVM_FORCE_HEAD_REVISION

Unless we're targeting Android, Linux builders don't have the NDK checked
out.

BUG=452726
NOTRY=true

Review URL: https://codereview.chromium.org/927703002

Cr-Commit-Position: refs/heads/master@{#316316}

tools/clang/scripts/update.sh

@@ -156,6 +156,11 @@ if [[ -n ${LLVM_FORCE_HEAD_REVISION:-''} ]]; then
   # Skip local patches when using HEAD: they probably don't apply anymore.
   with_patches=
 
+  if ! [[ "$GYP_DEFINES" =~ .*OS=android.* ]]; then
+    # Only build the Android ASan rt when targetting Android.
+    with_android=
+  fi
+
   echo "LLVM_FORCE_HEAD_REVISION was set; using r${CLANG_REVISION}"
 fi