Remove two fields from `SubstFolder`.

[nnethercote]

They're only used in error messages printed if there's an internal
compiler error, and the cost of maintaining them is high enough to show
up in profiles.

r? @matthewjasper

[nnethercote]

Some local measurements:

wf-projection-stress-65510-che...
        avg: -1.6%      min: -2.5%      max: -0.0%
ctfe-stress-4-check
        avg: -0.3%?     min: -0.5%?     max: -0.0%?
packed-simd-check
        avg: -0.2%      min: -0.3%      max: -0.0%
regression-31157-check
        avg: -0.1%      min: -0.3%      max: -0.0%
futures-check
        avg: -0.1%      min: -0.3%      max: -0.0%
wg-grammar-check
        avg: -0.2%      min: -0.2%      max: -0.0%
piston-image-check
        avg: -0.1%      min: -0.2%      max: -0.0%
ripgrep-check
        avg: -0.1%      min: -0.2%      max: 0.0%
serde-check
        avg: -0.1%      min: -0.2%      max: -0.0%
hyper-2-check
        avg: -0.1%      min: -0.2%      max: -0.0%
cargo-check
        avg: -0.1%      min: -0.2%      max: -0.0%

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion

[bors]

⌛ Trying commit ff51bdc with merge 0b19afe...

[bors]

☀️ Try build successful - checks-actions, checks-azure
Build commit: 0b19afe (0b19afee13894f423bd57e571688ba5631ec40ce)

[rust-timer]

Queued 0b19afe with parent 80f84eb, future comparison URL.

[matthiaskrgr]

Did you mean to r? matthewjasper perhaps?

[nnethercote]

Did you mean to r? matthewjasper perhaps?

GitHub suggested you as a reviewer, but it sounds like that wasn't a good suggestion.

r? @matthewjasper

[matthiaskrgr]

That's strange, I don't think I am in the reviewers list/have r+ powers 😅

[rust-timer]

Finished benchmarking try commit (0b19afe): 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

[matthewjasper]

@bors r+

[bors]

📌 Commit eeb4b83 has been approved by matthewjasper

[bors]

⌛ Testing commit eeb4b83 with merge d08eb98...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: matthewjasper
Pushing d08eb98 to master...