Clang update.py: set -DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON

TBR=thakis

Bug: 929645
Change-Id: Ie38f68f99b82e8193afbf367a7894f70cb9e18ec
Reviewed-on: https://chromium-review.googlesource.com/c/1458182Reviewed-by: Hans Wennborg <hans@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#629951}

tools/clang/scripts/update.py

@@ -538,6 +538,8 @@ def UpdateClang(args):
                      '-DCLANG_PLUGIN_SUPPORT=OFF',
                      '-DCLANG_ENABLE_STATIC_ANALYZER=OFF',
                      '-DCLANG_ENABLE_ARCMT=OFF',
+                     # TODO(crbug.com/929645): Use newer toolchain to host.
+                     '-DLLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN=ON',
                      ]
 
   if sys.platform != 'win32':