[SwiftWarningControl] Fix exponential queue growth in addWarningGroupControls

[inju2403]

Problem

addWarningGroupControls walks the diagnostic-group inheritance tree via BFS to propagate a control to every transitive sub-group. The implementation has two issues:

1. O(N²) per BFS: Array.removeFirst() shifts the rest of the queue on every dequeue.
2. No de-dup at dequeue: processedGroups is only checked when enqueuing sub-groups. A group reachable from more than one already-queued parent is enqueued multiple times, has its control
   re-applied each time, and (more importantly) re-expands its own sub-groups each time it's processed.

For a diamond-shaped inheritance DAG (each level has 2 nodes, both pointing to both nodes of the next level), the queue grows as 2^depth, so even modest depths (>= 10) effectively hang.

Fix

• Use an index-based queue instead of Array.removeFirst().
• De-duplicate at dequeue using processedGroups.insert(_).inserted.

Enqueue-time filtering is kept as an optimization, but it isn't sufficient on its own: between the moment a sub-group is filtered and the moment it would be dequeued, another path through the inheritance tree can still enqueue the same sub-group. The processedGroups.insert(_).inserted check at dequeue is what actually guarantees each group is processed at most once.

The loop now processes each distinct sub-group exactly once (the enqueue-time filter still admits duplicates that the dequeue-time check then skips, so the bound is O(N + E) where E is the number of edges in the inheritance graph). This matches the insert(_).inserted pattern already used elsewhere in this module (DiagnosticGroupInheritanceTree.hasCycle) and elsewhere in the repo (SwiftOperators/PrecedenceGraph, SwiftSyntax/Raw/RawSyntaxArena), and does not introduce any new dependencies.

Note: I left the enabledGroups.insert(diagnosticGroupIdentifier) line referencing the outer loop variable rather than groupIdentifier. Whether sub-groups that inherit a non-ignored control should also appear in enabledGroups looks like a separate semantic question that I'd rather not bundle into this perf fix -- happy to address it in a follow-up if reviewers prefer that semantic.

Measurements

Tests/PerformanceTest/WarningControlRegionsPerformanceTests.swift measures three workloads using _InstructionCounter:

workload	BEFORE	AFTER	speedup
chain depth=800	22.4M instr	14.5M instr	1.55x
fan width=1600	41.4M instr	27.8M instr	1.49x
diamond depth=8	5.27M instr	0.54M instr	9.81x
diamond depth>=10	effectively hangs	terminates	unbounded

A regression test (testDiamondSubGroupInheritance) is added to the default test suite to pin down the expected behavior on a diamond-shaped inheritance graph (perf tests are gated behind SWIFTSYNTAX_ENABLE_LONG_TESTS).

All 27 existing SwiftWarningControlTest cases pass.