Avoid a crash when using external link support with certain unexpected inputs.

[d-ronnqvist]

Bug/issue #, if applicable: rdar://148504975

Summary

This avoids a crash by accessing the identifier that the path hierarchy internally uses as a key rather than the identifier that's saved on the node. These should always be equal but some unexpected input can cause the node to have a different identifier than what the path hierarchy uses to find that node. The root cause of that issue still remains. This only avoids a crash that arises as a symptoms of the real issue.

Dependencies

None.

Testing

Because the root cause of this issue is still unknown, I can't provide better testing instructions than:

• Run docc convert --enable-experimental-external-link-support with some project that currently reproduces this crash.

Checklist

It's not possible to write an automated test for this change because any inputs necessary to create a path hierarchy where a node has a different identifier than the key in the lookup would fail to construct due to this assertion:

assert(
    lookup.allSatisfy({ $0.key == $0.value.identifier }),
    "Every node lookup should match a node with that identifier."
)

• [ ] Added tests See above.
• Ran the ./bin/test script and it succeeded
• [ ] Updated documentation if necessary Not applicable

[d-ronnqvist]

@swift-ci please test

[QuietMisdreavus]

Looks great, thanks!