[flang] Make sure the length is propagated when emboxing a char to unlimited polymoprhic box

When passing a character with unknown length to a subroutine expecting
an unlimited polymorphic pointer, a new descriptor is created. The
fir.embox operation needs to carry over the length from the character
to be passed correctly.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D144488