[AST] Mark the fallthrough coreturn statement implicit. (#77465)

This is a followup of #77311.

Author: hokein

clang/lib/Sema/SemaCoroutine.cpp

@@ -1748,7 +1748,7 @@ bool CoroutineStmtBuilder::makeOnFallthrough() {
       return false;
   } else if (HasRVoid) {
     Fallthrough = S.BuildCoreturnStmt(FD.getLocation(), nullptr,
-                                      /*IsImplicit*/false);
+                                      /*IsImplicit=*/true);
     Fallthrough = S.ActOnFinishFullStmt(Fallthrough.get());
     if (Fallthrough.isInvalid())
       return false;

clang/test/AST/ast-dump-coroutine.cpp

@@ -52,8 +52,7 @@ Task test()  {
 // CHECK-NEXT:       |-CXXMemberCallExpr  {{.*}} 'std::suspend_always'
 // CHECK-NEXT:       |   | `-MemberExpr {{.*}} .initial_suspend
 //                   ...
-// FIXME: the CoreturnStmt should be marked as implicit
-// CHECK: CoreturnStmt {{.*}} <col:6>{{$}}
+// CHECK: CoreturnStmt {{.*}} <col:6> implicit
 
 Task test2()  {
 // Writen souce code, verify no implicit bit for the co_return expr.
@@ -65,5 +64,4 @@ Task test2()  {
 // CHECK:        |-DeclStmt {{.*}}
 // CHECK-NEXT:   | `-VarDecl {{.*}} implicit used __promise
 //               ...
-// FIXME: the CoreturnStmt should be marked as implicit
-// CHECK: CoreturnStmt {{.*}} <col:6>{{$}}
+// CHECK: CoreturnStmt {{.*}} <col:6> implicit