IRGen: Reenable caching opaque type metadata accessors.

Dynamic replacement can only really get away with replacing opaque return types if a function's
opaque type has never been instantiated in the first place. Meanwhile, repeatedly instantiating
the metadata is too slow for many clients to tolerate. Fixes rdar://problem/53213600.

Author: jckarter

lib/IRGen/MetadataRequest.cpp

@@ -642,11 +642,6 @@ static MetadataResponse emitNominalMetadataRef(IRGenFunction &IGF,
 bool irgen::isTypeMetadataAccessTrivial(IRGenModule &IGM, CanType type) {
   assert(!type->hasArchetype());
 
-  // Support dynamically replacing opaque result types. Don't cache the
-  // accessor result.
-  if (!IGM.getOptions().shouldOptimize() && type->hasOpaqueArchetype())
-    return true;
-
   // Value type metadata only requires dynamic initialization on first
   // access if it contains a resilient type.
   if (isa<StructType>(type) || isa<EnumType>(type)) {
@@ -1489,6 +1484,7 @@ void irgen::emitCacheAccessFunction(IRGenModule &IGM,
 
   // If the load yielded null, emit the type metadata.
   IGF.Builder.emitBlock(isNullBB);
+  
   MetadataResponse response = getValue(IGF, parameters);
 
   // Ensure that we have a dynamically-correct state value.

test/IRGen/opaque_result_type.swift

@@ -161,10 +161,10 @@ public func useFoo(x: String, y: C) {
 }
 
 // CHECK-LABEL: define {{.*}} @"$s18opaque_result_type6useFoo1x1yySS_AA1CCtF"
-// CHECK: [[CONFORMANCE:%.*]] = call swiftcc i8** @swift_getOpaqueTypeConformance(i8* {{.*}}, %swift.type_descriptor* {{.*}}s18opaque_result_type3baz1zQrx_tAA1PRzAA1QRzlFQOMQ{{.*}}, [[WORD:i32|i64]] 2)
-// CHECK: [[MD:%.*]] = call swiftcc %swift.metadata_response @swift_getOpaqueTypeMetadata([[WORD]] 255, i8* {{.*}}, %swift.type_descriptor* {{.*}}s18opaque_result_type3baz1zQrx_tAA1PRzAA1QRzlFQOMQ{{.*}})
+// CHECK: [[CONFORMANCE:%.*]] = call swiftcc i8** @swift_getOpaqueTypeConformance(i8* {{.*}}, %swift.type_descriptor* {{.*}}s18opaque_result_type3baz1zQrx_tAA1PRzAA1QRzlFQOMQ{{.*}}, [[WORD:i32|i64]] 1)
+// CHECK: [[MD:%.*]] = call swiftcc %swift.metadata_response @"$s18opaque_result_type3baz1zQrx_tAA1PRzAA1QRzlFQOyAA1CCQo_Ma"
 // CHECK: [[TYPE:%.*]] = extractvalue %swift.metadata_response [[MD]], 0
-// CHECK: call swiftcc %swift.metadata_response @swift_getAssociatedTypeWitness([[WORD]] 0, i8** [[CONFORMANCE]], %swift.type* [[TYPE]]
+// CHECK: call swiftcc i8** @swift_getAssociatedConformanceWitness(i8** [[CONFORMANCE]], %swift.type* [[TYPE]]
 
 // CHECK-LABEL: define {{.*}} @"$sSS18opaque_result_type1PAA1AAaBP_AA1OPWT"
 // CHECK: call swiftcc i8** @swift_getOpaqueTypeConformance(i8* {{.*}}, %swift.type_descriptor* {{.*}}sSS18opaque_result_typeE3pooQryFQOMQ{{.*}}, [[WORD]] 1)