coverage: Consolidate FFI types into one module

[Zalathar]

Coverage FFI types were historically split across two modules, because some of them were needed by code in rustc_codegen_ssa.

Now that all of the coverage codegen code has been moved into rustc_codegen_llvm (#113355), it's possible to move all of the FFI types into a single module, making it easier to see all of them at once.

---

This PR only moves code and adjusts imports; there should be no functional changes.

[rustbot]

r? @oli-obk

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[Zalathar]

@rustbot label +A-code-coverage

[Zalathar]

I chose to move code out of llvm::ffi and into coverageinfo::ffi for a few reasons:

• llvm::ffi is already huge, so the types are easier to browse in their own smaller file
• There's only one type to actually move, plus a bunch of its constructors
• It feels slightly weird to have those constructors taking up vertical space in llvm::ffi

[oli-obk]

@bors r+ rollup

[bors]

📌 Commit d6ed6e3 has been approved by oli-obk

It is now in the queue for this repository.

[bors]

⌛ Testing commit d6ed6e3 with merge 5cbfee5...

[bors]

☀️ Test successful - checks-actions
Approved by: oli-obk
Pushing 5cbfee5 to master...

[rust-timer]

Finished benchmarking commit (5cbfee5): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was 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)	-0.6%	[-0.6%, -0.6%]	1
All ❌✅ (primary)	-	-	0

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)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.7%	[-2.7%, -2.7%]	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)	-3.3%	[-3.3%, -3.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.3%	[-3.3%, -3.3%]	1

Binary size

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

Bootstrap: 650.605s -> 648.769s (-0.28%)