[flang][hlfir] Inherit constant length for the result of hlfir.transpose.

Character length may be unknown for the type of Fortran::evaluate::FunctionRef
expression, but we can try to propagate it from the argument of
TRANSPOSE if it is known constant. Alternatively, we could relax
hlfir.transpose verification (i.e. allow character types mismatch
for the argument and the result).

Depends on D155912

Reviewed By: tblah

Differential Revision: https://reviews.llvm.org/D155913