[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;

include/swift/AST/TypeCheckRequests.h

@@ -2295,21 +2295,22 @@ public:
 
 /// Builds an opaque result type for a declaration.
 class OpaqueResultTypeRequest
-    : public SimpleRequest<OpaqueResultTypeRequest,
-                           OpaqueTypeDecl *(ValueDecl *),
-                           RequestFlags::Cached> {
+    : public SimpleRequest<
+          OpaqueResultTypeRequest, OpaqueTypeDecl *(ValueDecl *),
+          RequestFlags::SeparatelyCached | RequestFlags::SplitCached> {
 public:
   using SimpleRequest::SimpleRequest;
 
 private:
   friend SimpleRequest;
 
-  OpaqueTypeDecl *
-  evaluate(Evaluator &evaluator, ValueDecl *VD) const;
+  OpaqueTypeDecl *evaluate(Evaluator &evaluator, ValueDecl *VD) const;
 
 public:
-  // Caching.
+  // Split caching.
   bool isCached() const { return true; }
+  std::optional<OpaqueTypeDecl *> getCachedResult() const;
+  void cacheResult(OpaqueTypeDecl *result) const;
 };
 
 /// Determines if a function declaration is 'static'.

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -262,7 +262,7 @@ SWIFT_REQUEST(TypeChecker, OpaqueReadOwnershipRequest,
               NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, OpaqueResultTypeRequest,
               OpaqueTypeDecl *(ValueDecl *),
-              Cached, NoLocationInfo)
+              SeparatelyCached | SplitCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, OperatorPrecedenceGroupRequest,
               PrecedenceGroupDecl *(PrecedenceGroupDecl *),
               Cached, NoLocationInfo)