FIR lowering of the fir.box type inside fir.global is special (it is an
actual descriptor struct value instead of being a descriptor in memory)
and causes builtin.unrealized_conversion_cast to be inserted under the
hood by MLIR dialect conversion framework after each operation producing
a fir.box is translated. These builtin.unrealized_conversion_cast must
be removed before the code generation of operation of using the fir.box
in order to get the right "by value" code generation required in global
initial value definitions.
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