SILGen: Fix generated vtable thunk when a final override is more visible than the base [5.1]

[slavapestov]

Don't re-dispatch to the override's vtable slot if the override
is final; there's no vtable slot and this will result in an
infinite loop.

Fixes rdar://problem/52006394.

[slavapestov]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 98295375b5a1a8b54e581ba3beb51665ddf5e24a

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 98295375b5a1a8b54e581ba3beb51665ddf5e24a

[tkremenek]

The test failure looks like a minor difference in expected output:

// CHECK-NEXT: #Base.privateMethod3!1: <T> (Base<T>) -> (Int) -> () : @$s17vtables_multifile12FinalDerivedC14privateMethod3yySiSgFAA4BaseCAD33_63E5F2521A3C787F5F9EFD57FB9237EALLyySiFTV [override] // vtable thunk for Base.privateMethod3(_:) dispatching to FinalDerived.privateMethod3(_:)
               ^
<stdin>:872:2: note: scanning from here
 #Base.privateMethod3!1: <T> (Base<T>) -> (Int) -> () : hidden @$s17vtables_multifile12FinalDerivedC14privateMethod3yySiSgFAA4BaseCAD33_63E5F2521A3C787F5F9EFD57FB9237EALLyySiFTV [override] // vtable thunk for Base.privateMethod3(_:) dispatching to FinalDerived.privateMethod3(_:)

Note the hidden after the <T> (Base<T>) -> (Int) -> () : which is not in the expected output.

[slavapestov]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 98295375b5a1a8b54e581ba3beb51665ddf5e24a

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 98295375b5a1a8b54e581ba3beb51665ddf5e24a