`common_reference` isn't an exception for [meta.rqmts]/4, so it's better
to disallow users to specialize it.
`indirectly_readable.compile.pass.cpp` was a bit problematic. It
attempted to opt-out common reference type in some wrong ways. Also, the
standard effectively forbids opting-out common reference type for `T&`
and `T&&`. This patch removes and adjusts some problematic cases.
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Co-authored-by: Nikolas Klauser <nikolasklauser@berlin.de>
fdb11c1be6