[NFC] IRGen: Vend a FunctionPointer of this helper

lib/IRGen/GenFunc.cpp

@@ -904,6 +904,13 @@ llvm::Constant *irgen::getCoroFrameAllocStubFn(IRGenModule &IGM) {
       /*optionalLinkageOverride=*/nullptr, llvm::CallingConv::C);
 }
 
+FunctionPointer irgen::getCoroFrameAllocStubFunctionPointer(IRGenModule &IGM) {
+  auto *fn = cast<llvm::Function>(getCoroFrameAllocStubFn(IGM));
+  auto sig = Signature::forFunction(fn);
+  return FunctionPointer::forDirect(FunctionPointerKind::Function, fn, nullptr,
+                                    sig);
+}
+
 static Size getOffsetOfOpaqueIsolationField(IRGenModule &IGM,
                                       const LoadableTypeInfo &isolationTI) {
   auto offset = IGM.RefCountedStructSize;

lib/IRGen/GenFunc.h

@@ -65,6 +65,7 @@ namespace irgen {
   /// Stub function that weakly links againt the swift_coroFrameAlloc
   /// function. This is required for back-deployment.
   llvm::Constant *getCoroFrameAllocStubFn(IRGenModule &IGM);
+  FunctionPointer getCoroFrameAllocStubFunctionPointer(IRGenModule &IGM);
 } // end namespace irgen
 } // end namespace swift