[flang] add API to copy and update descriptors for assumed ranks (#93305)

When passing assumed-rank around, the lower bounds, dynamic type and
attribute must sometimes be updated to match the dummy attributes. See
https://github.com/llvm/llvm-project/blob/main/flang/docs/AssumedRank.md#annex-1---descriptor-temporary-for-the-dummy-arguments
for more details.

Doing it inline would require generating many instructions and block CFG
at the LLVM IR dialect level in codegen. Go for a simple runtime API
instead.
A matching fir.rebox_assumed_rank operation will be created and will
allow for easier future optimizations when inlining is done in FIR.