[flang] Add fir.box_typecode operation

`fir.box_typecode` operation allows to retrieve the type code
from a boxed value. This will be used in the `fir.select_type` conversion
to if-then-else ladder for type guard statement with intrinsic type spec
instead of using a runtime call.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D137829