[Sema] Stop filtering nested protocol types from lookup

This check wasn't ever correct, because the fact that the the protocol comes
from another module doesn't change the fact that the type is valid for lookup
within this module. It incorrectly rejects the following, valid code:

```swift
// In A.swift
public protocol A {}
```

```
// In B.swift
import A

extension A {
  typealias B = Int
  func b(_ b: Self.B) {}
}
```

Author: Harlan Haskins

lib/AST/GenericSignatureBuilder.cpp

@@ -2046,17 +2046,8 @@ TypeDecl *EquivalenceClass::lookupNestedType(
         continue;
       }
 
-      // If this is another type declaration, determine whether we should
-      // record it.
+      // If this is another type declaration, record it.
       if (auto type = dyn_cast<TypeDecl>(member)) {
-        // FIXME: Filter out type declarations that aren't in the same
-        // module as the protocol itself. This is an unprincipled hack, but
-        // provides consistent lookup semantics for the generic signature
-        // builder in all contents.
-        if (type->getDeclContext()->getParentModule()
-              != proto->getParentModule())
-          continue;
-
         concreteDecls.push_back(type);
         continue;
       }

test/Generics/Inputs/external-protocol.swift

@@ -0,0 +1 @@
+public protocol AnExternalProtocol {}

test/Generics/typealias-in-extension-of-external-protocol.swift

@@ -0,0 +1,12 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -emit-module-path %t/ExternalProtocol.swiftmodule %S/Inputs/external-protocol.swift -module-name ExternalProtocol
+// RUN: %target-swift-frontend -typecheck -I %t %s
+
+import ExternalProtocol
+
+extension AnExternalProtocol {
+  typealias TypeAlias = Int
+
+  func methodUsingAlias(_ alias: Self.TypeAlias) {}
+}