Better error for for<...> on associated type bound

[compiler-errors]

With GATs just around the corner, we'll probably see more people trying out Trait<for<'a> Assoc<'a> = ..>.

This PR improves the syntax error slightly, and also makes it slightly easier to make this into real syntax in the future.

Feel free to push back if the reviewer thinks this should have a suggestion on how to fix it (i.e. push the for<'a> outside of the angle brackets), but that can also be handled in a follow-up PR.

[rust-highfive]

r? @nagisa

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[nagisa]

r=me after the comments are resolved

[compiler-errors]

@bors r=nagisa

[bors]

📌 Commit b65265b has been approved by nagisa

[bors]

⌛ Testing commit b65265b with merge 48a9e10...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa
Pushing 48a9e10 to master...

[rust-timer]

Finished benchmarking commit (48a9e10): comparison url.

Summary:

• Primary benchmarks: no relevant changes found
• Secondary benchmarks: 😿 relevant regressions found

	Regressions 😿 (primary)	Regressions 😿 (secondary)	Improvements 🎉 (primary)	Improvements 🎉 (secondary)	All 😿 🎉 (primary)
count1	0	4	0	0	0
mean2	N/A	0.4%	N/A	N/A	N/A
max	N/A	0.4%	N/A	N/A	N/A

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

@rustbot label: -perf-regression

Footnotes

1. number of relevant changes ↩

2. the arithmetic mean of the percent change ↩