[flang] Use fir.type_desc and delay type desc address resolution

Makes use of fir.type_desc in order to delay the type desc address
resolution. The lowering inserts fir.type_desc operation instead of fir.addr_of
operation pointing to the fir.global type descriptor. The fir.type_desc
operation is then lowered in code gen to an address of operation in the LLVM
dialect. At this stage, the type descriptor is generated in all cases.

Reviewed By: vdonaldson

Differential Revision: https://reviews.llvm.org/D142920