MissingGenericFunctionDefinition sorted poorly in test output

[danakj]

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Description of the bug:

The MissingGenericFunctionDefinition error ends up being attached to test output far below where it is emitted if there are other errors below. It seems to always get sorted to last for some reason?

@zygoloid suggests we may want a custom sort key for the diagnostic.

fn C[T:! type](t: T);

fn F() {
  C({});

  // CHECK:STDERR: fail_impl_cycle_one_generic_param.carbon:[[@LINE+4]]:3: error: name `a` not found [NameNotFound]
  // CHECK:STDERR:   a;
  // CHECK:STDERR:   ^
  // CHECK:STDERR:
  a;
  // CHECK:STDERR: fail_impl_cycle_one_generic_param.carbon:[[@LINE+11]]:3: error: name `b` not found [NameNotFound]
  // CHECK:STDERR:   b;
  // CHECK:STDERR:   ^
  // CHECK:STDERR:
  // CHECK:STDERR: fail_impl_cycle_one_generic_param.carbon:[[@LINE-11]]:3: error: use of undefined generic function [MissingGenericFunctionDefinition]
  // CHECK:STDERR:   C({});
  // CHECK:STDERR:   ^
  // CHECK:STDERR: fail_impl_cycle_one_generic_param.carbon:[[@LINE-17]]:1: note: generic function declared here [MissingGenericFunctionDefinitionHere]
  // CHECK:STDERR: fn C[T:! type](t: T);
  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~
  // CHECK:STDERR:
  b;
}

[danakj]

Filing this bug because I don't intend to work on this right now - if anyone wants to pick it up just assign it.

[jonmeow]

This is because these issues are checked as part of CheckRequiredDefinitions. They would need to be given more specific last nodes in their diagnostic locations. As a reminder, this is a consequence of #4778.

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