[flang] Fold instantiated PDT character component length when needed

In case a character component PDT length only depends on kind parameters,
fold it while instantiating the PDT. This is especially important if the
component has an initializer because later semantic phases (offset
computation or runtime type info generation) might get confused and
generate offset/type info that will lead to crashes in lowering.

Differential Revision: https://reviews.llvm.org/D122938