[clang] [clang] fix the unexpected controlflow in `ParseTentative.cpp` (PR #95917)

[Nikita Popov via cfe-commits]

nikic wrote:

The TryParseProtocolQualifiers code path can be reached using something like this:
```
struct X {
};
void foo() {
  X<P>;
}
```
But I don't know how to make that valid code without it taking some different code path...

https://github.com/llvm/llvm-project/pull/95917