SDBMExpr: factor uniqu'ing out of MLIRContext.cpp

    SDBM expressions are designed as components of an attribute, similarly to
    affine expressions.  As such, they need to be unique'd in the MLIRContext.
    When SDBM expressions were implemented, uniqu'ing objects in a context required
    to modify MLIRContext implementation.  This is no longer the case as generic
    StorageUniquer has been introduced.  Port the SDBMExpr uniqu'ing to use a newly
    introduced uniquer and remove SDBM construction from MLIRContext.cpp.

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PiperOrigin-RevId: 249244739