[flang] Add support for type-bound generics

Record the bindings in a symbol with GenericBindingDetails, including
bindings inherited from extended types. Check that they have consistent
accessibility.

Original-commit: flang-compiler/f18@0f780abcea829d807493cf6a71eab9d8cffd4673
Reviewed-on: https://github.com/flang-compiler/f18/pull/249