[flang] Fix folding of character array kind conversion

The folding of conversions of character arrays from one kind to
another was not preserving the value of the character length expression
on the intermediate array constructor, causing spurious errors
when defining named constant arrays of one kind of character with
arrays of another kind.

Fixes llvm-test-suite/Fortran/gfortran/regression/widechar_10.f90.

Differential Revision: https://reviews.llvm.org/D157340