AffineExprVisitor: fix names of default visitation functions.

Existing default visitation function for dimension and symbols were called
"visitAffineDimExpr" and "visitAffineSymbolExpr".  However, generic CRTP-based
visit and walk methods were calling "visitDimExpr" and "visitSymbolExpr",
respectively, on derived classes.  This has not been discovered before because
all existing affine expression visitors (re)define functions for dimensions and
symbols.  Change the names of the default empty visitation functions to the
latter form.

PiperOrigin-RevId: 222312114