rustc_expand: Switch FxHashMap to FxIndexMap where iteration is used

[NiklasJonsson]

Relates #84447

[rust-highfive]

r? @compiler-errors

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[NiklasJonsson]

I tried to run rustc_perf locally to see if there was any performance impact like in this MR but I couldn't get xperf to run (I'm on windows), so it would probably good to do a perf run to see if this has any impact.

[petrochenkov]

This map is used purely for diagnostics, so it's hot performance sensitive.

[compiler-errors]

@bors r+

[bors]

📌 Commit b47a934 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit b47a934 with merge 47ba935...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing 47ba935 to master...

[rust-timer]

Finished benchmarking commit (47ba935): comparison url.

Instruction count

• Primary benchmarks: 🎉 relevant improvements found
• Secondary benchmarks: 😿 relevant regression found

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	0.4%	0.4%	1
Improvements 🎉 (primary)	-1.1%	-1.8%	11
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	-1.1%	-1.8%	11

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

Results

• Primary benchmarks: 🎉 relevant improvement found
• Secondary benchmarks: mixed results

	mean1	max	count2
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	2.7%	2.7%	1
Improvements 🎉 (primary)	-3.1%	-3.1%	1
Improvements 🎉 (secondary)	-2.3%	-2.3%	1
All 😿🎉 (primary)	-3.1%	-3.1%	1

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

1. the arithmetic mean of the percent change ↩ ↩²

2. number of relevant changes ↩ ↩²