[test] Test covariant erasure for constrained existentials

Author: angela-laar

test/type/parameterized_existential.swift

@@ -92,8 +92,14 @@ func protocolCompositionNotSupported1(_: SomeProto & Sequence<Int>) {}
 func protocolCompositionNotSupported2(_: any SomeProto & Sequence<Int>) {}
 // expected-error@-1 {{non-protocol, non-class type 'Sequence<Int>' cannot be used within a protocol-constrained type}}
 
-func increment(n : any Sequence<Float>) {
+func increment(_ n : any Collection<Float>) {
   for value in n {
       _ = value + 1
   }
 }
+
+func genericIncrement<T: Numeric>(_ n : any Collection<T>) {
+  for value in n {
+    _ = value + 1
+  }
+}