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This prevents the compiler from adding the copy of libc++ installed
alongside clang to the include path, which can result in very confusing
error messages because both copies of libc++ have a stddef.h which
will try to #include_next the system one, resulting in no stddef.h
being included at all due to a header guard clash.

Normally there will not be a copy of libc++ installed alongside clang
because it's not included in the clang package on Linux, but folks
with a custom clang_base_path may end up with one.

Change-Id: I6734c0335db486b30ee7ec849687fb8064a358b1
Reviewed-on: https://chromium-review.googlesource.com/c/1343532Reviewed-by: agrieve <agrieve@chromium.org>
Commit-Queue: Peter Collingbourne <pcc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609594}