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

[via cfe-commits]

================
@@ -1743,7 +1743,7 @@ bool CoroutineStmtBuilder::makeOnFallthrough() {
       return false;
   } else if (HasRVoid) {
     Fallthrough = S.BuildCoreturnStmt(FD.getLocation(), nullptr,
-                                      /*IsImplicit*/false);
+                                      /*IsImplicit*/true);
----------------
cor3ntin wrote:

```suggestion
                                      /*IsImplicit=*/true);
```

https://github.com/llvm/llvm-project/pull/77465