Remove DefPath from Visibility and calculate it on demand

[jyn514]

Depends on #80090 and should not be merged before. Helps with #79103 and #76382.

cc #80014 (comment) - @nnethercote I figured it out! It was simpler than I expected :)

This brings the size of clean::Visibility down from 40 bytes to 8.

Note that this does not remove clean::Visibility, even though it's now basically the same as ty::Visibility, because the Invsible variant means something different from Inherited and I thought it would be be confusing to merge the two. See the new comments on impl Clean for ty::Visibility for details.

[rust-highfive]

r? @ollie27

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[jyn514]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

[bors]

⌛ Trying commit a2fb4b9 with merge fbe5dedd7d90604c50736a818a6ea08e195a377e...

[bors]

☀️ Try build successful - checks-actions
Build commit: fbe5dedd7d90604c50736a818a6ea08e195a377e (fbe5dedd7d90604c50736a818a6ea08e195a377e)

[rust-timer]

Queued fbe5dedd7d90604c50736a818a6ea08e195a377e with parent 353f3a3, future comparison URL.

@rustbot label: +S-waiting-on-perf

[rust-timer]

Finished benchmarking try commit (fbe5dedd7d90604c50736a818a6ea08e195a377e): comparison url.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. Please note that if the perf results are neutral, you should likely undo the rollup=never given below by specifying rollup- to bors.

Importantly, though, if the results of this run are non-neutral do not roll this PR up -- it will mask other regressions or improvements in the roll up.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf

[jyn514]

-.5% on instructions, -.6% on max-rss.

@bors r=GuillaumeGomez rollup=never

[bors]

📌 Commit a2fb4b9 has been approved by GuillaumeGomez

[bors]

⌛ Testing commit a2fb4b9 with merge 28d73a3...

[bors]

☀️ Test successful - checks-actions
Approved by: GuillaumeGomez
Pushing 28d73a3 to master...