[SE-0258] Fix crash-on-invalid involving some wrappers with wrapperValue

Fixes rdar://problem/51193573.

Author: DougGregor

lib/Sema/CodeSynthesis.cpp

@@ -1734,7 +1734,7 @@ PropertyWrapperBackingPropertyInfoRequest::evaluate(Evaluator &evaluator,
                                                      VarDecl *var) const {
   // Determine the type of the backing property.
   auto wrapperType = var->getPropertyWrapperBackingPropertyType();
-  if (!wrapperType)
+  if (!wrapperType || wrapperType->hasError())
     return PropertyWrapperBackingPropertyInfo();
 
   auto wrapperInfo = var->getAttachedPropertyWrapperTypeInfo();

test/decl/var/property_wrappers.swift

@@ -714,3 +714,20 @@ struct TestPD {
   @PD(a: "foo") var foo: Int = 42 // expected-error{{property 'foo' with attached wrapper cannot initialize both the wrapper type and the property}}
   // expected-error@-1{{missing argument for parameter 'initialValue' in call}}
 }
+
+protocol P { }
+
+@_propertyWrapper
+struct WrapperRequiresP<T: P> {
+  var value: T
+  var wrapperValue: T { return value }
+}
+
+struct UsesWrapperRequiringP {
+  // expected-note@-1{{in declaration of}}
+  
+  @WrapperRequiresP var x.: UsesWrapperRequiringP
+  // expected-error@-1{{expected member name following '.'}}
+  // expected-error@-2{{expected declaration}}
+  // expected-error@-3{{type annotation missing in pattern}}
+}