[AutoDiff] Enable ownership for JVP/VJP SILGen thunks. (#27393)
Enable ownership for JVP/VJP SILGen thunks.
Ownership verification ensures that these thunks do not leak memory.
Delete redundant helper `forwardFunctionArgumentsConvertingOwnership`,
which is no longer necessary.
Various thunk leak-checking tests already exist in
test/AutoDiff/leakchecking.swift.
Resolves TF-703: all AutoDiff SILGen thunks now use ownership.
