[IRGen] Sign these function pointers.
Value witness tables for prespecialized metadata for multi payload enums
contain references to `swift_getMultiPayloadEnumTagSinglePayload` and
`swift_storeMultiPayloadEnumTagSinglePayload`. On platforms with
ptrauth, those functions must be signed. Use the same helper when
adding these functions to the table as is used to add every single other
function to the table.
rdar://80334865
