[IRGen] Define coroutine function pointer types.

They're isomorphic to async function pointers.  Even so, having this
distinction will be clearer than reusing AFPs for this other purpose.

Author: nate-chandler

lib/IRGen/IRGenModule.cpp

@@ -783,6 +783,10 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
 
   DifferentiabilityWitnessTy = createStructType(
       *this, "swift.differentiability_witness", {Int8PtrTy, Int8PtrTy});
+
+  CoroFunctionPointerTy = createStructType(*this, "swift.coro_func_pointer",
+                                           {RelativeAddressTy, Int32Ty}, true);
+  CoroFunctionPointerPtrTy = CoroFunctionPointerTy->getPointerTo();
 }
 
 IRGenModule::~IRGenModule() {

lib/IRGen/IRGenModule.h

@@ -842,6 +842,9 @@ class IRGenModule {
   llvm::StructType *DifferentiabilityWitnessTy; // { i8*, i8* }
   // clang-format on
 
+  llvm::StructType *CoroFunctionPointerTy; // { i32, i32 }
+  llvm::PointerType *CoroFunctionPointerPtrTy;
+
   llvm::GlobalVariable *TheTrivialPropertyDescriptor = nullptr;
 
   llvm::Constant *swiftImmortalRefCount = nullptr;