[flang] Allow forward reference to binding name in type-bound generic

Type-bound generic statements can have binding names that refer to
bindings that occur later in the type. So save a map of generic to
binding names and process them when we get to the end of the
type-bound procedure part. This is similar to how specific procedures
of generic identifiers are handled.

Also detect duplicate binding names for a type-bound generic.

Fixes issue flang-compiler/f18#572.

Original-commit: flang-compiler/f18@d58bb77cfa8cde2838a5eaba3e44956bdaa042e7
Reviewed-on: https://github.com/flang-compiler/f18/pull/577