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Previously we used a GCC/libstdc++ package from Ubuntu Precise to build a Clang
that would work on Precise or later. These days, all bots and developers are on
Trusty or later, so this should no longer be necessary.

Besides simplifying things, this allows us to easier move to using the
latest CMake when building Clang, because it was built with a newer
libstdc++ than the Precise one.

Bug: 797991
Change-Id: I528825389241f60ccc96167015370cd5e285725d
Reviewed-on: https://chromium-review.googlesource.com/1188679Reviewed-by: Reid Kleckner <rnk@chromium.org>
Commit-Queue: Hans Wennborg <hans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585953}