[6.0] Fix two bugs with opaque return types -vs- parameter packs #72853
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6.0 cherry-pick of key bits of #72847
Description: Fix infinite recursion in opaque type substitution when parameter packs are present, and a crash when opaque return type witnessed by variadic generic type with conditional conformance.
Reviewed by: @hborla
Risk: Low
Radar: rdar://123645784, rdar://125668798