Fix validation-test/Sema/OverridesAndOverloads.swift after compiler started produced more warnings

Author: gribozavr

validation-test/Sema/OverridesAndOverloads.swift

@@ -122,10 +122,10 @@ Overrides.test("contravariant return type override, optional to non-optional") {
     }
   }
 
-  Derived().foo() as C1
+  _ = Derived().foo() as C1
   expectEqual("Derived.foo() -> C1", which)
 
-  Derived().foo() as C1?
+  _ = Derived().foo() as C1?
   expectEqual("Derived.foo() -> C1", which)
 }
 
@@ -139,10 +139,10 @@ Overrides.test("contravariant return type override, base class to derived class"
     }
   }
 
-  Derived().foo() as C1
+  _ = Derived().foo() as C1
   expectEqual("Derived.foo() -> C1x", which)
 
-  Derived().foo() as C1x
+  _ = Derived().foo() as C1x
   expectEqual("Derived.foo() -> C1x", which)
 }
 
@@ -156,10 +156,10 @@ Overrides.test("contravariant return type override, optional base class to non-o
     }
   }
 
-  Derived().foo() as C1
+  _ = Derived().foo() as C1
   expectEqual("Derived.foo() -> C1x", which)
 
-  Derived().foo() as C1x
+  _ = Derived().foo() as C1x
   expectEqual("Derived.foo() -> C1x", which)
 }
 
@@ -180,7 +180,7 @@ Overrides.test("contravariant return type override, protocol to protocol") {
   // Derived().foo() as P1 // error: ambiguous use of 'foo()'
   // expectEqual("Derived.foo() -> P1x", which)
 
-  Derived().foo() as P1x
+  _ = Derived().foo() as P1x
   expectEqual("Derived.foo() -> P1x", which)
 }
 
@@ -201,7 +201,7 @@ Overrides.test("contravariant return type override, protocol to struct") {
   // Derived().foo() as P1 // error: ambiguous use of 'foo()'
   // expectEqual("Derived.foo() -> P1ImplS1", which)
 
-  Derived().foo() as P1ImplS1
+  _ = Derived().foo() as P1ImplS1
   expectEqual("Derived.foo() -> P1ImplS1", which)
 }