ASTMangler: sink fix to be more safe
My previous fix in `6f6a46f` was the correct fix in theory, but in
practice it could accidentally change the mangling of something I
haven't considered, which would break ABI with Swift 6.0
I've narrowed that fix here to only affect dependent conformances
specifically for Copyable/Escapable. The existing code in
`appendDependentProtocolConformance` would always reach a trap because
we're mangling a conformance path that ends with Copyable/Escapable.
We can assume no such symbol has been successfully been mangled before,
thanks to the pre-existing skip in `conformanceRequirementIndex`, so
there's no risk of ABI change.
rdar://135310019
