Do this even if the function then contains references to other functions with wrong linkage.
MandatoryPerformanceOptimization fixes the linkage afterwards.
This is similar to what we already do with de-virtualizing class and witness methods: https://github.com/swiftlang/swift/pull/76032
rdar://168171608
In regular swift this is a nice optimization. In embedded swift it's a requirement, because the compiler needs to be able to specialize generic deinits of non-copyable types.
The new de-virtualization utilities are called from two places:
* from the new DeinitDevirtualizer pass. It replaces the old MoveOnlyDeinitDevirtualization, which is very basic and does not fulfill the needs for embedded swift.
* from MandatoryPerformanceOptimizations for embedded swift