Detemplatize convertRegion in DialectConversion

    Originally, FunctionConverter::convertRegion in the DialectConversion framework
    was implemented as a function template because it was creating a new region in
    the parent object, which could have been an op or a function.  Since
    DialectConversion now operates in place, new region is no longer created so
    there is no need for convertRegion to be aware of the parent, only of the error
    reporting location.

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PiperOrigin-RevId: 249826392