[Sema] Better handle recovery for structurally invalid ReturnStmts

Make sure we preserve the result expression for an out-of-place
`return`, or a non-`nil` result in an initializer. This ensures we
can still provide semantic functionality from them and fixes a crash
where we would fail to type-check a binding.

test/stmt/statements.swift

@@ -179,6 +179,8 @@ return 42 // expected-error {{return invalid outside of a func}}
 
 return // expected-error {{return invalid outside of a func}}
 
+return VoidReturn1() // expected-error {{return invalid outside of a func}}
+
 func NonVoidReturn1() -> Int {
   _ = 0
   return // expected-error {{non-void function should return a value}}