AffineExpr: factor uniqu'ing out of MLIRContext.cpp

    Affine expressions are designed as components of an attribute and are unique'd
    in the MLIRContext.  When affine 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
    AffineExpr construction to use the new infrastructure by introducing an
    affineUniquer into the MLIRContext.

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PiperOrigin-RevId: 249207539