### What does this PR do?
The DevSever's `IncrementalGraph` uses a data-oriented design memory
management style, storing data in lists and using indices instead of
pointers.
In conventional memory management, when we free a pointer and
accidentally use it will trip up asan. Obviously this doesn't apply when
using lists and indices, so this PR adds a check in debug & asan builds.
Everytime we free an `Edge` we better make sure that there are no more
dangling references to that spot.
This caught a case where we weren't setting `g.first_import[file_index]
= .none` when deleting a file's imports, causing a dangling reference
and out of bounds access.
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Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
