[Serialization] Make requirement signature conformance loading lazy.

While the original commit was expected to have no effective impact, it
turned out to actually solve some circularity issues after all
involving protocols that inherit from other protocols. It should be a
mild compile-time win as well in cases where the requirement signature
is not needed at all.

Test in subsequent commit.

https://bugs.swift.org/browse/SR-5191 / rdar://problem/33804554

(cherry picked from commit 897effe)

Author: jrose-apple

lib/Serialization/Deserialization.cpp

@@ -647,59 +647,6 @@ NormalProtocolConformance *ModuleFile::readNormalConformance(
   dc->getAsNominalTypeOrNominalTypeExtensionContext()
     ->registerProtocolConformance(conformance);
 
-  // Read requirement signature conformances.
-  SmallVector<ProtocolConformanceRef, 4> reqConformances;
-
-  if (proto->isObjC() && getContext().LangOpts.EnableDeserializationRecovery) {
-    // Don't crash if inherited protocols are added or removed.
-    // This is limited to Objective-C protocols because we know their only
-    // conformance requirements are on Self. This isn't actually a /safe/ change
-    // even in Objective-C, but we mostly just don't want to crash.
-
-    // FIXME: DenseMap requires that its value type be default-constructible,
-    // which ProtocolConformanceRef is not, hence the extra Optional.
-    llvm::SmallDenseMap<ProtocolDecl *, Optional<ProtocolConformanceRef>, 16>
-        conformancesForProtocols;
-    while (conformanceCount--) {
-      ProtocolConformanceRef nextConformance = readConformance(DeclTypeCursor);
-      ProtocolDecl *confProto = nextConformance.getRequirement();
-      conformancesForProtocols[confProto] = nextConformance;
-    }
-
-    for (const auto &req : proto->getRequirementSignature()) {
-      if (req.getKind() != RequirementKind::Conformance)
-        continue;
-      ProtocolDecl *proto =
-          req.getSecondType()->castTo<ProtocolType>()->getDecl();
-      auto iter = conformancesForProtocols.find(proto);
-      if (iter != conformancesForProtocols.end()) {
-        reqConformances.push_back(iter->getSecond().getValue());
-      } else {
-        // Put in an abstract conformance as a placeholder. This is a lie, but
-        // there's not much better we can do. We're relying on the fact that
-        // the rest of the compiler doesn't actually need to check the
-        // conformance to an Objective-C protocol for anything important.
-        // There are no associated types and we don't emit a Swift conformance
-        // record.
-        reqConformances.push_back(ProtocolConformanceRef(proto));
-      }
-    }
-
-  } else {
-    auto isConformanceReq = [](const Requirement &req) {
-      return req.getKind() == RequirementKind::Conformance;
-    };
-    if (conformanceCount != llvm::count_if(proto->getRequirementSignature(),
-                                           isConformanceReq)) {
-      fatal(llvm::make_error<llvm::StringError>(
-          "serialized conformances do not match requirement signature",
-          llvm::inconvertibleErrorCode()));
-    }
-    while (conformanceCount--)
-      reqConformances.push_back(readConformance(DeclTypeCursor));
-  }
-  conformance->setSignatureConformances(reqConformances);
-
   // If the conformance is complete, we're done.
   if (conformance->isComplete())
     return conformance;
@@ -4633,10 +4580,59 @@ void ModuleFile::finishNormalConformance(NormalProtocolConformance *conformance,
                                               typeCount, conformanceCount,
                                               rawIDs);
 
-  // Skip requirement signature conformances.
-  auto proto = conformance->getProtocol();
-  while (conformanceCount--)
-    (void)readConformance(DeclTypeCursor);
+  // Read requirement signature conformances.
+  const ProtocolDecl *proto = conformance->getProtocol();
+  SmallVector<ProtocolConformanceRef, 4> reqConformances;
+
+  if (proto->isObjC() && getContext().LangOpts.EnableDeserializationRecovery) {
+    // Don't crash if inherited protocols are added or removed.
+    // This is limited to Objective-C protocols because we know their only
+    // conformance requirements are on Self. This isn't actually a /safe/ change
+    // even in Objective-C, but we mostly just don't want to crash.
+
+    // FIXME: DenseMap requires that its value type be default-constructible,
+    // which ProtocolConformanceRef is not, hence the extra Optional.
+    llvm::SmallDenseMap<ProtocolDecl *, Optional<ProtocolConformanceRef>, 16>
+        conformancesForProtocols;
+    while (conformanceCount--) {
+      ProtocolConformanceRef nextConformance = readConformance(DeclTypeCursor);
+      ProtocolDecl *confProto = nextConformance.getRequirement();
+      conformancesForProtocols[confProto] = nextConformance;
+    }
+
+    for (const auto &req : proto->getRequirementSignature()) {
+      if (req.getKind() != RequirementKind::Conformance)
+        continue;
+      ProtocolDecl *proto =
+          req.getSecondType()->castTo<ProtocolType>()->getDecl();
+      auto iter = conformancesForProtocols.find(proto);
+      if (iter != conformancesForProtocols.end()) {
+        reqConformances.push_back(iter->getSecond().getValue());
+      } else {
+        // Put in an abstract conformance as a placeholder. This is a lie, but
+        // there's not much better we can do. We're relying on the fact that
+        // the rest of the compiler doesn't actually need to check the
+        // conformance to an Objective-C protocol for anything important.
+        // There are no associated types and we don't emit a Swift conformance
+        // record.
+        reqConformances.push_back(ProtocolConformanceRef(proto));
+      }
+    }
+
+  } else {
+    auto isConformanceReq = [](const Requirement &req) {
+      return req.getKind() == RequirementKind::Conformance;
+    };
+    if (conformanceCount != llvm::count_if(proto->getRequirementSignature(),
+                                           isConformanceReq)) {
+      fatal(llvm::make_error<llvm::StringError>(
+          "serialized conformances do not match requirement signature",
+          llvm::inconvertibleErrorCode()));
+    }
+    while (conformanceCount--)
+      reqConformances.push_back(readConformance(DeclTypeCursor));
+  }
+  conformance->setSignatureConformances(reqConformances);
 
   ArrayRef<uint64_t>::iterator rawIDIter = rawIDs.begin();