AST: Fix excessive deserialization in GenericSignatureBuilder

When we're looking up all associated types with the same name in order
to find the right archetype anchor, skip extension members to avoid
circular deserialization.

Discovered while investigating <rdar://problem/30248571>.

lib/AST/GenericSignatureBuilder.cpp

@@ -554,7 +554,16 @@ auto GenericSignatureBuilder::PotentialArchetype::getNestedType(
   SmallVector<std::pair<ProtocolDecl *, RequirementSource>, 4>
     conformsTo(rep->ConformsTo.begin(), rep->ConformsTo.end());
   for (auto &conforms : conformsTo) {
-    for (auto member : conforms.first->lookupDirect(nestedName)) {
+    // Make sure we don't trigger deserialization of extensions,
+    // since they can refer back to a protocol we're currently
+    // type checking.
+    //
+    // Note that typealiases in extensions won't matter here,
+    // because a typealias is never going to be a representative
+    // PA.
+    auto members = conforms.first->lookupDirect(nestedName,
+                                                /*ignoreNewExtensions=*/true);
+    for (auto member : members) {
       PotentialArchetype *pa;
 
       if (auto assocType = dyn_cast<AssociatedTypeDecl>(member)) {

test/Serialization/Inputs/circular-associated-type/a.swift

@@ -0,0 +1,3 @@
+public protocol A {
+  associatedtype T : B
+}

test/Serialization/Inputs/circular-associated-type/b.swift

@@ -0,0 +1,7 @@
+public protocol BB {
+  associatedtype T
+}
+
+public protocol B {
+  associatedtype T : BB
+}

test/Serialization/Inputs/circular-associated-type/c.swift

@@ -0,0 +1,5 @@
+extension B {
+  public init?<T : A>(_: T) where T.T == Self {
+    return nil
+  }
+}

test/Serialization/multi-file-associated-type-circularity.swift

@@ -0,0 +1,8 @@
+// RUN: rm -rf %t && mkdir -p %t
+
+// RUN: %target-swift-frontend -emit-module -module-name Multi -o %t/a.swiftmodule -primary-file %S/Inputs/circular-associated-type/a.swift %S/Inputs/circular-associated-type/b.swift %S/Inputs/circular-associated-type/c.swift
+// RUN: %target-swift-frontend -emit-module -module-name Multi -o %t/b.swiftmodule -primary-file %S/Inputs/circular-associated-type/b.swift %S/Inputs/circular-associated-type/a.swift %S/Inputs/circular-associated-type/c.swift
+// RUN: %target-swift-frontend -emit-module -module-name Multi -o %t/c.swiftmodule -primary-file %S/Inputs/circular-associated-type/c.swift %S/Inputs/circular-associated-type/a.swift %S/Inputs/circular-associated-type/b.swift
+
+// RUN: %target-swift-frontend -parse-as-library -emit-module -module-name Multi %t/a.swiftmodule %t/b.swiftmodule %t/c.swiftmodule -o %t
+