Re-apply "AST: Fix excessive deserialization in GenericSignatureBuilder"

This got reverted because it made our non-deterministic deserialization
crasher come up more often. Re-applying this patch to test the theory
that @jrose-apple's fix addressed the issue.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -1142,9 +1142,17 @@ auto GenericSignatureBuilder::PotentialArchetype::getNestedType(
   SmallVector<std::pair<ProtocolDecl *, const RequirementSource *>, 4>
     conformsTo(rep->ConformsTo.begin(), rep->ConformsTo.end());
   for (auto &conforms : conformsTo) {
-    auto proto = conforms.first;
-
-    for (auto member : proto->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 *proto = conforms.first;
+    auto members = proto->lookupDirect(nestedName,
+                                       /*ignoreNewExtensions=*/true);
+    for (auto member : members) {
       PotentialArchetype *pa;
       std::function<void(Type, Type)> diagnoseMismatch;
 

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
+