Move SDBM uniquer into the SDBM dialect

    MLIRContext does not have to be aware of the SDBM unique data structures
    directly.  Move the SDBM storage uniquer from MLIRContext to the SDBM dialect
    instance.  Expressions that previously required a context to be constructed now
    require an instance of the dialect in order to access the uniquer.  While they
    could look up the dialect in the context, it would have introduced a rather
    expensive lookup into each construction.  Instead, the caller is expected to
    obtain the dialect instance and cache it.

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PiperOrigin-RevId: 249245199