[IRGen] Fix missing musttail for dynamic replacement calls.

Fixes rdar://78284346.

(Found when trying to land musttail verification. :D)

Author: varungandhi-apple

lib/IRGen/GenDecl.cpp

@@ -2955,6 +2955,10 @@ void IRGenModule::emitDynamicReplacementOriginalFunctionThunk(SILFunction *f) {
       FunctionPointer(fnType, typeFnPtr, authInfo, signature)
           .getAsFunction(IGF),
       forwardedArgs);
+  Res->setTailCall();
+  if (f->isAsync()) {
+    Res->setTailCallKind(IGF.IGM.AsyncTailCallKind);
+  }
 
   if (implFn->getReturnType()->isVoidTy())
     IGF.Builder.CreateRetVoid();

test/IRGen/async_dynamic_replacement.swift

@@ -14,8 +14,7 @@ internal func _replacement_number() async -> Int {
 // rdar://78284346 - Dynamic replacement should use musttail
 //                   for tail calls from swifttailcc to swifttailcc
 // CHECK-LABEL: define {{.*}} swifttailcc void @"$s25async_dynamic_replacement01_C7_numberSiyYaFTI"
-// CHECK-NOT: musttail
-// CHECK: call swifttailcc void
+// CHECK: musttail call swifttailcc void
 // CHECK-NEXT: ret void
 
 public func calls_number() async -> Int {