[flang] Allow fir.class in AnyRefOrBox

Some operations are using `AnyRefOrBox` to specify the type of the
operands or attribute. This is the case for the `fir.coordinate_of`
operation. This patch updates the `AnyRefOrBox` to accept `BaseBoxType`
instead of only `BoxType`.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D135442