IRGen: Emit labeled tuple and function type metadata containing pack expansions [5.9]

[slavapestov]

• Main PRs: IRGen: Support tuple labels in emitDynamicTupleTypeMetadataRef() #67573 IRGen: Emitting metadata for function types containing pack expansion parameters #67724
• Description: IRGen didn't know how to emit metadata for tuples with labels, or function types, containing pack expansions.
• Scope of the issue: For example, we would crash if you tried to call a generic function func f<U>(_: U) with a substitution U := (repeat each T, label: Int) where T is a type parameter pack in the caller's context.
• Risk: Medium; there's a fair bit of new code here that runs in the variadic case, and some refactoring that moves existing code around. However I think this is an important case to support.
• Tested: Executable tests added.
• Reviewed by: @nate-chandler or @rjmccall

[slavapestov]

@swift-ci Please test