[compiletest] Add more test ignore reasons, needs- validation, and improved error messages

[pietroalbini]

This PR makes more improvements to the way compiletest ignoring headers are handled, following up on #108905:

• Human-readable ignore reasons have been added for the remaining ignore causes (needs-* directives, *llvm* directives, and debugger version directives). All ignored tests should now have a human-readable reason.
• The code handling needs-* directives has been refactored, and now invalid needs-* directive emit errors like ignore-* and only-*.
• All errors are now displayed at startup (with line numbers) rather than just the first error of the first file.

This PR is best reviewed commit-by-commit.

r? @ehuss

[ehuss]

Nice!

Again I have not validated the before/after ignore count, but from a cursory look at all the ignored tests it looks good.

@bors r+

[bors]

📌 Commit 34a52df has been approved by ehuss

It is now in the queue for this repository.

[bors]

⌛ Testing commit 34a52df with merge 50b816f...

[bors]

☀️ Test successful - checks-actions
Approved by: ehuss
Pushing 50b816f to master...

[rust-timer]

Finished benchmarking commit (50b816f): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-1.3%	[-1.3%, -1.3%]	1
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1