[IRGen] Disable on-stack pack metadata via attr.

When lowering a SIL function, if it bears the no_onstack_pack_metadata
annotation, disable on-stack pack metadata.

Author: nate-chandler

lib/IRGen/IRGenFunction.h

@@ -201,10 +201,12 @@ class IRGenFunction {
   llvm::Value *AsyncCoroutineCurrentResume = nullptr;
   llvm::Value *AsyncCoroutineCurrentContinuationContext = nullptr;
 
+protected:
   // Whether pack metadata stack promotion is disabled for this function in
   // particular.
   bool packMetadataStackPromotionDisabled = false;
 
+private:
   Address asyncContextLocation;
 
   /// The unique block that calls @llvm.coro.end.

lib/IRGen/IRGenSIL.cpp

@@ -2355,6 +2355,9 @@ void IRGenSILFunction::emitSILFunction() {
   if (IGM.DebugInfo)
     IGM.DebugInfo->emitFunction(*CurSILFn, CurFn);
 
+  if (!CurSILFn->useStackForPackMetadata())
+    packMetadataStackPromotionDisabled = true;
+
   // Map the entry bb.
   LoweredBBs[&*CurSILFn->begin()] = LoweredBB(&CurFn->back(), {});
   // Create LLVM basic blocks for the other bbs.