[flang][hlfir] Support type descriptor for initialized character component

These compiler generated component descriptor include designators packaged
as CLASS(*) for simplicity. HLFIR hit an assert in an std::get trying to
recover an Expr<SomeChar> while translating the expression type.
Use the dynamic type of the CLASS(*) expr in that case to recover the
compiler length.

Differential Revision: https://reviews.llvm.org/D144960