[flang] Support CHARACTER(4) pointer targets

fir.rebox is emitting an llvm.sdiv to compute the character length
given the byte size from the input descriptor.
Inside a fir.global, this is not needed given the target length must
be accessible via the type, and it caused MLIR to fail LLVM IR
code generation (and crash).

Use the input type length when available instead.

Reviewed By: PeteSteinfeld, vzakhari

Differential Revision: https://reviews.llvm.org/D154072