fix ambiguous/wrong origins for calls from bridge methods

[codecholeric]

Originally we assumed that comparing declaring class, method name and parameter type names would be good enough to identify the caller of a method, since the compiler does not allow to declare two methods with the same name and parameter types inside the same class. However, the compiler might add methods with the same name and parameter types in certain cases (bridge methods).
Consider

class Parent {
  Object method() {
    return null;
  }
}

class Child extends Parent {
  @Override
  String method() {
    return null;
  }
}

To implement covariant return types (the child overriding the method using a subtype as return type) the compiler transparently adds a so-called "bridge method" that has the original type as return type (i.e. overrides 1-2-1) and simply forwards to the one with the subtype as return type.
We now take the full descriptor (containing parameter types and return types) to match the origin of a caller. By that the ambiguity is resolved, since the descriptor/full signature actually has to be unique.

Resolves: #513