We had a GCC+ASan+UBSan full test run on both the default configuration and
the full configuration. In the full configuration, use Clang instead.
Empirically, a modern Clang's UBSan finds things that GCC's doesn't.
Insist on a modern Clang (by default, our CI would pick the oldest
available, which doesn't find as much stuff). Define "modern" as the most
recent that's available on our CI right now, which is Ubuntu 20.04.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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