[Sema] Add note to all unresolved witness types instead of just the first one. (#3458)

When issuing the error of "not conforming to a protocol P", we used to only note the first
unresolved witness type. This is inconsistent with the situation when the conformance
fails due to unimplemented functions, which we note all of the unimplemented functions.
This patch fixed this by noting all unresolved witness types.

Author: nkcsgexi

lib/Sema/TypeCheckProtocol.cpp

@@ -3413,16 +3413,14 @@ void ConformanceChecker::resolveTypeWitnesses() {
       return;
     }
 
-    // Complain that we couldn't find anything.
-    if (!missingTypeWitness)
-      missingTypeWitness = *unresolvedAssocTypes.begin();
-
-    diagnoseOrDefer(missingTypeWitness, true,
-      [missingTypeWitness](TypeChecker &tc,
-                           NormalProtocolConformance *conformance) {
-      tc.diagnose(missingTypeWitness, diag::no_witnesses_type,
-                  missingTypeWitness->getName());
-    });
+    for (auto missingTypeWitness : unresolvedAssocTypes) {
+      diagnoseOrDefer(missingTypeWitness, true,
+        [missingTypeWitness](TypeChecker &tc,
+                             NormalProtocolConformance *conformance) {
+          tc.diagnose(missingTypeWitness, diag::no_witnesses_type,
+                      missingTypeWitness->getName());
+        });
+    }
 
     return;
   }

test/decl/ext/protocol.swift

@@ -899,3 +899,11 @@ class BadClass1 : BadProto1 {
   func foo() { }
   override var prop: Int { return 5 } // expected-error{{property does not override any property from its superclass}}
 }
+
+protocol BadProto5 {
+  associatedtype T1 // expected-note{{protocol requires nested type 'T1'}}
+  associatedtype T2 // expected-note{{protocol requires nested type 'T2'}}
+  associatedtype T3 // expected-note{{protocol requires nested type 'T3'}}
+}
+
+class BadClass5 : BadProto5 {} // expected-error{{type 'BadClass5' does not conform to protocol 'BadProto5'}}