Two fixes here.
* Prevent a crash when sorting the list of analysis errors when some
errors refer to lost source locations. These errors can be sorted
anywhere in the list, because they are (in theory) guaranteed to never
be emitted by the `resolveReferences` logic. This case occurs, for
instance, when a declaration has compile errors in the initial update
and is deleted in the second update.
* Prevent a crash when resolving the source location for `entire_file`
errors for a non-existent file. This is the bug underlying #20954.
Resolves: #20954.
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