scanner: Suppress odr errors from asan

By default asan complains if the same symbol has multiple definitions. This
will happen if a binary links against both the client and server libraries,
as they both contain a copy of the core protocol. This is okay because
(assuming their versions match) the definitions will be exactly the
same.

Formalize this by adding suppression annotations to the generated
code when running with public visibility. If someone links against two
copies of the same protocol privately, that's more likely to be a real
error, so leave that unsuppressed.

Signed-off-by: Fergus Dall <sidereal@google.com>
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