[flang][FIR] add fir.is_assumed_size operation (#93853)

Assumed-rank fir.box/class may describe assumed-size array. This case
needs special handling in SELECT RANK. It is not possible to generate
FIR code to detect that a fir.box is an assumed-size (the way to detect
that is to check that upper dimension extent is -1 in the descriptor).

Instead of emitting a runtime call directly in lowering, add an
operation that can later be lowered to a runtime call or inline code
when the descriptor layout is known.