[Mangling] Only look for retroactive conformances in conditional reqs

Previously, the mangler searched for retroactive conformances in /any/
of a generic type's substitutions, but really we only care about the
ones that affect the generic type's conformance, i.e. those that
affect generic parameters. Refining this results in shorter mangled
names involving instantiations of generic types.

Follow-up work for rdar://problem/46735592

Author: jrose-apple

lib/AST/ASTMangler.cpp

@@ -1137,16 +1137,31 @@ static bool containsRetroactiveConformance(
                                       const ProtocolConformance *conformance,
                                       ModuleDecl *module) {
   // If the root conformance is retroactive, it's retroactive.
-  if (isRetroactiveConformance(conformance->getRootConformance()))
+  const RootProtocolConformance *rootConformance =
+      conformance->getRootConformance();
+  if (isRetroactiveConformance(rootConformance))
     return true;
 
-  // If any of the substitutions used to form this conformance are retroactive,
-  // it's retroactive.
+  // If the conformance is conditional and any of the substitutions used to
+  // satisfy the conditions are retroactive, it's retroactive.
   auto subMap = conformance->getSubstitutions(module);
-  for (auto conformance : subMap.getConformances()) {
-    if (conformance.isConcrete() &&
-        containsRetroactiveConformance(conformance.getConcrete(), module))
+  for (auto requirement : rootConformance->getConditionalRequirements()) {
+    if (requirement.getKind() != RequirementKind::Conformance)
+      continue;
+    ProtocolDecl *proto =
+        requirement.getSecondType()->castTo<ProtocolType>()->getDecl();
+    Optional<ProtocolConformanceRef> conformance =
+        subMap.lookupConformance(requirement.getFirstType()->getCanonicalType(),
+                                 proto);
+    if (!conformance) {
+      // This should only happen when mangling invalid ASTs, but that happens
+      // for indexing purposes.
+      continue;
+    }
+    if (conformance->isConcrete() &&
+        containsRetroactiveConformance(conformance->getConcrete(), module)) {
       return true;
+    }
   }
 
   return false;

test/SILGen/mangling_retroactive.swift

@@ -48,7 +48,7 @@ struct RequiresP<T: P> {}
 // UnconditionallyP, even though UnconditionallyP's generic param uses a
 // retroactive conformance to conform to Q.
 func rdar46735592(_: RequiresP<UnconditionallyP<Y>>) { }
-// CHECK: sil hidden [ossa] @$s20mangling_retroactive12rdar46735592yyAA9RequiresPVyAA16UnconditionallyPVy12RetroactiveB1YVAI0F1A1QAAHPyHCg_GAlJ1PyHCg_GF
+// CHECK: sil hidden [ossa] @$s20mangling_retroactive12rdar46735592yyAA9RequiresPVyAA16UnconditionallyPVy12RetroactiveB1YVAI0F1A1QAAHPyHCg_GGF
 
 struct QImpl: Q {}
 struct ConditionallyP<T> {}