[clang] [clang] fix the unexpected control flow in ParseTentative.cpp (PR #109298)

[via cfe-commits]

cor3ntin wrote:

> @cor3ntin See the discussion on #95917. This is likely effectively dead code, but nobody is certain enough of that to delete it outright...

The original issue seems to suggest the code path can be used on invalid code such as 
````
struct X {
};
void foo() {
  X<P>;
}
```

(ie a bogus template-name that is not annotated)
We might test that - and see that the errors make a bit of sense/don't regress

https://github.com/llvm/llvm-project/pull/109298