[Concurrency] More cleanups for getCurrentAsyncTask builtin.

Michael has ghostwritten all of this to address his own comments.
Thank you!

Author: DougGregor

lib/SIL/IR/OperandOwnership.cpp

@@ -1033,9 +1033,16 @@ ANY_OWNERSHIP_BUILTIN(IntInstrprofIncrement)
   }
 CONSTANT_OWNERSHIP_BUILTIN(Owned, MustBeInvalidated, COWBufferForReading)
 CONSTANT_OWNERSHIP_BUILTIN(Owned, MustBeInvalidated, UnsafeGuaranteed)
-CONSTANT_OWNERSHIP_BUILTIN(Owned, MustBeInvalidated, GetCurrentAsyncTask)
 #undef CONSTANT_OWNERSHIP_BUILTIN
 
+#define SHOULD_NEVER_VISIT_BUILTIN(ID)                              \
+  OperandOwnershipKindMap OperandOwnershipKindBuiltinClassifier::visit##ID(    \
+      BuiltinInst *, StringRef) {                                              \
+    llvm_unreachable("Builtin should never be visited! E.x.: It may not have arguments"); \
+  }
+SHOULD_NEVER_VISIT_BUILTIN(GetCurrentAsyncTask)
+#undef SHOULD_NEVER_VISIT_BUILTIN
+
 // Builtins that should be lowered to SIL instructions so we should never see
 // them.
 #define BUILTIN_SIL_OPERATION(ID, NAME, CATEGORY)                              \

lib/SILGen/SILGenBuiltin.cpp

@@ -1402,8 +1402,7 @@ static ManagedValue emitBuiltinGetCurrentAsyncTask(
       loc,
       ctx.getIdentifier(getBuiltinName(BuiltinValueKind::GetCurrentAsyncTask)),
       SGF.getLoweredType(ctx.TheNativeObjectType), SubstitutionMap(), { });
-  SGF.enterEndLifetimeCleanup(apply);
-  return ManagedValue::forUnmanaged(apply);
+  return SGF.emitManagedRValueWithEndLifetimeCleanup(apply);
 }
 
 Optional<SpecializedEmitter>

lib/SILGen/SILGenDecl.cpp

@@ -1347,9 +1347,10 @@ class EndLifetimeCleanup : public Cleanup {
 };
 } // end anonymous namespace
 
-CleanupHandle SILGenFunction::enterEndLifetimeCleanup(SILValue value) {
+ManagedValue SILGenFunction::emitManagedRValueWithEndLifetimeCleanup(
+    SILValue value) {
   Cleanups.pushCleanup<EndLifetimeCleanup>(value);
-  return Cleanups.getTopCleanup();
+  return ManagedValue::forUnmanaged(value);
 }
 
 namespace {

lib/SILGen/SILGenFunction.h

@@ -2001,8 +2001,19 @@ class LLVM_LIBRARY_VISIBILITY SILGenFunction
   /// Enter a cleanup to emit a ReleaseValue/DestroyAddr of the specified value.
   CleanupHandle enterDestroyCleanup(SILValue valueOrAddr);
 
-  /// Enter cleanup to emit an EndLifetime operation for the given value.
-  CleanupHandle enterEndLifetimeCleanup(SILValue value);
+  /// Return an owned managed value for \p value that is cleaned up using an end_lifetime instruction.
+  ///
+  /// The end_lifetime cleanup is not placed into the ManagedValue itself and
+  /// thus can not be forwarded. This means that the ManagedValue is treated
+  /// as a +0 value. This means that the owned value will be copied by SILGen
+  /// if it is ever needed as a +1 value (meaning any time that the value
+  /// escapes).
+  ///
+  /// DISCUSSION: end_lifetime ends the lifetime of an owned value in OSSA
+  /// without resulting in a destroy being emitted. This cleanup should only
+  /// be used for owned values that do not need to be destroyed if they do not
+  /// escape the current call frame but need to be copied if they escape.
+  ManagedValue emitManagedRValueWithEndLifetimeCleanup(SILValue value);
 
   /// Enter a cleanup to emit a DeinitExistentialAddr or DeinitExistentialBox
   /// of the specified value.