Revert "Convert ActorIsolationRequest to use split caching"

There are a few spots in `ActorIsolationRequest` that produce
`unspecified` isolation with `preconcurrency` bit set and diagnostics
rely on that to make downgrade decisions in Swift 5 mode.

This reverts commit 0097ef6.

Author: xedin

include/swift/AST/Decl.h

@@ -2828,10 +2828,6 @@ class ValueDecl : public Decl {
     /// optional result.
     unsigned isIUO : 1;
 
-    /// Whether we're in the common case where the ActorIsolationRequest
-    /// request returned ActorIsolation::forUnspecified().
-    unsigned noActorIsolation : 1;
-
     /// Whether we've evaluated the ApplyAccessNoteRequest.
     unsigned accessNoteApplied : 1;
   } LazySemanticInfo = { };
@@ -2847,6 +2843,7 @@ class ValueDecl : public Decl {
   friend class ActorIsolationRequest;
   friend class DynamicallyReplacedDeclRequest;
   friend class ApplyAccessNoteRequest;
+
   friend class Decl;
   SourceLoc getLocFromSource() const { return NameLoc; }
 protected:

include/swift/AST/TypeCheckRequests.h

@@ -1537,8 +1537,7 @@ class GlobalActorAttributeRequest
 class ActorIsolationRequest :
     public SimpleRequest<ActorIsolationRequest,
                          ActorIsolation(ValueDecl *),
-                         RequestFlags::SeparatelyCached |
-                         RequestFlags::SplitCached> {
+                         RequestFlags::Cached> {
 public:
   using SimpleRequest::SimpleRequest;
 
@@ -1548,10 +1547,8 @@ class ActorIsolationRequest :
   ActorIsolation evaluate(Evaluator &evaluator, ValueDecl *value) const;
 
 public:
-  // Separate.
+  // Caching
   bool isCached() const { return true; }
-  std::optional<ActorIsolation> getCachedResult() const;
-  void cacheResult(ActorIsolation value) const;
 };
 
 /// Determine whether the given function should have an isolated 'self'.

include/swift/AST/TypeCheckerTypeIDZone.def

@@ -178,7 +178,7 @@ SWIFT_REQUEST(TypeChecker, GlobalActorAttributeRequest,
               SeparatelyCached | SplitCached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, ActorIsolationRequest,
               ActorIsolationState(ValueDecl *),
-              SeparatelyCached | SplitCached, NoLocationInfo)
+              Cached, NoLocationInfo)
 SWIFT_REQUEST(TypeChecker, HasIsolatedSelfRequest,
               bool(ValueDecl *),
               Uncached, NoLocationInfo)

lib/AST/TypeCheckRequests.cpp

@@ -2013,28 +2013,6 @@ GlobalActorAttributeRequest::cacheResult(std::optional<CustomAttrNominalPair> va
   }
 }
 
-//----------------------------------------------------------------------------//
-// ActorIsolationRequest computation.
-//----------------------------------------------------------------------------//
-
-std::optional<ActorIsolation> ActorIsolationRequest::getCachedResult() const {
-  auto *decl = std::get<0>(getStorage());
-  if (decl->LazySemanticInfo.noActorIsolation)
-    return ActorIsolation::forUnspecified();
-
-  return decl->getASTContext().evaluator.getCachedNonEmptyOutput(*this);
-}
-
-void ActorIsolationRequest::cacheResult(ActorIsolation value) const {
-  auto *decl = std::get<0>(getStorage());
-  if (value.isUnspecified()) {
-    decl->LazySemanticInfo.noActorIsolation = 1;
-    return;
-  }
-
-  decl->getASTContext().evaluator.cacheNonEmptyOutput(*this, std::move(value));
-}
-
 //----------------------------------------------------------------------------//
 // ResolveMacroRequest computation.
 //----------------------------------------------------------------------------//