Fix the open-existential remover to not die on the OVEs in collection upcasts

lib/Sema/CSDiag.cpp

@@ -3136,6 +3136,17 @@ static void eraseOpenedExistentials(Expr *&expr) {
                "didn't see this OVE in a containing OpenExistentialExpr?");
         return { true, value->second };
       }
+
+      // Handle collection upcasts specially so that we don't blow up on
+      // their embedded OVEs.
+      if (auto CDE = dyn_cast<CollectionUpcastConversionExpr>(expr)) {
+        if (auto result = CDE->getSubExpr()->walk(*this)) {
+          CDE->setSubExpr(result);
+          return { false, CDE };
+        } else {
+          return { true, CDE };
+        }
+      }
 
       return { true, expr };
     }

validation-test/Sema/type_checker_crashers_fixed/rdar27815848.swift

@@ -0,0 +1,13 @@
+// RUN: not %target-swift-frontend %s -parse
+
+class C {}
+class D : C {}
+
+@_silgen_name("consume")
+func consume(_: [C])  // note, returns ()
+
+// Assert/crash while emitting diagnostic for coercion from () to Bool
+// in the context of a collection cast.
+func test(x: [D]) -> Bool {
+  return consume(x)  // no way to coerce from () to Bool
+}