[6.0] Heap allocate our atomics

[ahoppen]

• Explanation: We used C atomics but these were allocated as Swift variables. Even thought they were atomic, concurrent accesses to them could violate Swift’s exclusivity laws, raising thread sanitizer errors. Allocate the C atomic using malloc to fix this problem.
• Scope: Only affects assert builds
• Risk: Low, only affects assert builds
• Testing: Verified that we no longer hit a thread sanitizer issue in SourceKit-LSP with this change
• Issue: rdar://129170128
• Reviewer: @hamishknight on https://github.com/apple/swift-syntax/pull/2671

[ahoppen]

@swift-ci Please test