[flang] Fix fir.dispatch_table generation with derived-type parameter with kind type parameter

When generating the `fir.dispatch_table` operation, the name of the parent
derived-type needs to be mangled. For this the type instantiation
DerivedTypeSpec needs to be retrieved to have the correct kind type parameters.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D140209