[5.7][CodeCompletion] Maintain LeaveClosureBodiesUnchecked when checking pattern binding

[ahoppen]

Cherry-pick #58662 to release/5.7

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• Explanation: When type checking for code completion, we need to leave closure bodies unchecked (for reasons I don’t want to go into detail here). But when type checking part of the AST entered a pattern binding decl, it would forget about leaving closure bodies unchecked, causing us to miss completions.
• Scope: Code completion inside pattern binding decls
• Risk: Low (only affects code completion)
• Testing: Added regression test, ran SourceKit stress tester
• Issue: rdar://92603748 [Enum element labels not suggested in unresolved dot completion inside multi-statement closure #58610]
• Reviewer: @xedin on [CodeCompletion] Maintain LeaveClosureBodiesUnchecked when checking pattern binding #58662

[ahoppen]

@swift-ci Please test