Reject CVarArgs in parse_ty_for_where_clause

[fmease]

Fixes #125847. This regressed in #77035 where the parse_ty inside parse_ty_where_predicate was replaced with the at the time new parse_ty_for_where_clause which incorrectly stated it would permit CVarArgs (maybe a copy/paste error).

r? parser

[compiler-errors]

r=me when green

[fmease]

Ah, I forgot to mention that we're technically regressing stable code like #[cfg(any())] fn f3() where for<> ...: {} here. However, the likelihood that anybody relies on it is astronomically low (it requires a for</*...*/> in front of it because ... can't actually begin a type officially).

The MBE example I gave above is also unlikely to occur in practice but the chances are way higher (ppls' DSLs are crazy sometimes, cc #115785).

@bors r=compiler-errors

[bors]

📌 Commit 8938609 has been approved by compiler-errors

It is now in the queue for this repository.

[bors]

⌛ Testing commit 8938609 with merge f67a1ac...

[bors]

☀️ Test successful - checks-actions
Approved by: compiler-errors
Pushing f67a1ac to master...

[rust-timer]

Finished benchmarking commit (f67a1ac): comparison URL.

Overall result: ❌ regressions - 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)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.7%)

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)	-3.7%	[-3.7%, -3.7%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.7%	[-3.7%, -3.7%]	1

Cycles

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

Binary size

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

Bootstrap: 667.702s -> 668.08s (0.06%)
Artifact size: 318.95 MiB -> 318.88 MiB (-0.02%)