Fix a bug introduced by the recursive-protocol validation

special case by ensuring that protocols are fully validated
before Sema completes.

The test case doesn't currently fail without this patch, but
there are pending changes which will cause SILGen and IRGen
to depend on the requirement signature existing.

Author: rjmccall

lib/Sema/TypeCheckDecl.cpp

@@ -7302,6 +7302,9 @@ void TypeChecker::validateDeclForNameLookup(ValueDecl *D) {
         validateDeclForNameLookup(ATD);
       }
     }
+
+    // Make sure the protocol is fully validated by the end of Sema.
+    TypesToFinalize.insert(proto);
     break;
   }
   case DeclKind::AssociatedType: {

lib/Sema/TypeChecker.cpp

@@ -490,6 +490,15 @@ static void finalizeType(TypeChecker &TC, NominalTypeDecl *nominal) {
     TC.addImplicitConstructors(CD);
     TC.addImplicitDestructor(CD);
   }
+
+  // validateDeclForNameLookup will not trigger an immediate full
+  // validation of protocols, but clients will assume that things
+  // like the requirement signature have been set.
+  if (auto PD = dyn_cast<ProtocolDecl>(nominal)) {
+    if (!PD->isRequirementSignatureComputed()) {
+      TC.validateDecl(PD);
+    }
+  }
 }
 
 static void typeCheckFunctionsAndExternalDecls(TypeChecker &TC) {

test/IRGen/Inputs/witness_table_multifile_2.swift

@@ -9,3 +9,6 @@ struct X : P {
 }
 
 func go() -> P { return X("hello") }
+
+protocol ProtocolOnlyUsedAsAType {
+}

test/IRGen/witness_table_multifile.swift

@@ -11,3 +11,9 @@ func bar() {
   // CHECK-NEXT: getelementptr inbounds i8*, i8** [[WITNESS_TABLE]], i32 3
   go().foo()
 }
+
+// Ensure that protocols from other files get fully validated even
+// when they're only used as types.
+func useAProtocol() -> ProtocolOnlyUsedAsAType? {
+  return nil
+}