[flang] Improve length information in character transformational (#65771)

Intrinsic resolution currently does not resolve constant length
information for character transformational (with "sameChar") where the
argument has constant length but is not a variable or a constant
expression.

It is not required to fold those expressions (only inquiry on constant
expression or variable with constant length is required to be a constant
expression).

But constant length information for character is valuable for lowering,
so I think this is a nice and easy to have.