[AST] Use cached deserialized decl in getOpaqueResultTypeDecl

Mangling and looking up the opaque result type decl
for serialized decls is a fairly expensive
operation. Instead, fallthrough to the request
which will have a cached value set by deserialization.
This shaves ~30ms off the cached completion for:

```swift
import SwiftUI

struct V: View {
  var body: some View {
    Table(#^CC^#
  }
}
```

include/swift/AST/Decl.h

@@ -2758,6 +2758,10 @@ private:
     /// output a null pointer.
     unsigned noDynamicallyReplacedDecl : 1;
 
+    /// Whether the OpaqueResultTypeRequest request was evaluated and produced
+    /// a null pointer.
+    unsigned noOpaqueResultType : 1;
+
     /// Whether the "isFinal" bit has been computed yet.
     unsigned isFinalComputed : 1;
 
@@ -2785,7 +2789,7 @@ private:
   friend class InterfaceTypeRequest;
   friend class CheckRedeclarationRequest;
   friend class ActorIsolationRequest;
-  friend class DynamicallyReplacedDeclRequest;
+  friend class OpaqueResultTypeRequest;
   friend class ApplyAccessNoteRequest;
 
   friend class Decl;