[Constraint System] Fix covariant erasure for constrained existentials

Use of an associated type in the member is not considered invariant
if that associated type has been made concrete by the existential type.

Author: angela-laar

lib/Sema/ConstraintSystem.cpp

@@ -2103,7 +2103,7 @@ typeEraseExistentialSelfReferences(
       if (t->is<GenericTypeParamType>()) {
         erasedTy = baseTy;
       } else {
-        erasedTy = existentialSig->getNonDependentUpperBounds(t);
+        erasedTy = existentialSig->getDependentUpperBounds(t);
       }
 
       if (metatypeDepth) {

test/type/parameterized_existential.swift

@@ -91,3 +91,9 @@ 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>) {
+  for value in n {
+      _ = value + 1
+  }
+}