Don't use dep_graph.with_ignore when encoding fn param names

[Aaron1011]

The call to with_ignore was added years ago, and should no longer be
necessary with the modern incremental query infrastructure.

I also removed a related FIXME comment for issue #38501, which was
closed months ago.

[rust-highfive]

r? @lcnr

(rust_highfive has picked a reviewer for you, use r? to override)

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit 5d40e45 with merge 465b1652e593ffc0e226b5e06bd770104f1451b7...

[lcnr]

r? @eddyb

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 465b1652e593ffc0e226b5e06bd770104f1451b7 (465b1652e593ffc0e226b5e06bd770104f1451b7)

[rust-timer]

Queued 465b1652e593ffc0e226b5e06bd770104f1451b7 with parent e8df0b8, future comparison URL.

[rust-timer]

Finished benchmarking try commit (465b1652e593ffc0e226b5e06bd770104f1451b7): 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

[eddyb]

@bors r+

[bors]

📌 Commit 5d40e45 has been approved by eddyb

[bors]

⌛ Testing commit 5d40e45 with merge b287b56...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: eddyb
Pushing b287b56 to master...