[flang][hlfir] Fixed length-one assignment.

Assignment from a character dummy argument to a length-one character
variable resulted in illegal fir.convert:
  %0 = fir.load %unboxed_dummy : !fir.ref<!fir.char<1,?>>
  %1 = fir.convert %0 : (!fir.char<1,?>) -> !fir.char<1>
  fir.store %1 to %local : !fir.ref<!fir.char<1>>

This change fixes the length-one assignment code to use proper casts.

For character dummy arguments with constant length we will now also
type cast the unboxed reference to the character type with constant length
during the lowering:
  fir.convert %x : (!fir.ref<!fir.char<1,?>>) -> !fir.ref<!fir.char<1,8>>

I also adjusted the length-one assignment recognition so that in case
of same-length assignment we recognize length-one from either LHS or RHS
data types.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D149382