[Parse] Improve diagnostic and recovery when there is an extra override in the...

[Parse] Improve diagnostic and recovery when there is an extra override in the outline method definition.

The clang behavior was poor before this patch:

```
void B::foo() override {}
// Before: clang emited "expcted function body after function
// declarator", and skiped all contents until it hits a ";", the
// following function f() is discarded.

// VS

// Now "override is not allowed" with a remove fixit, and following f()
// is retained.
void f();
```

Differential Revision: https://reviews.llvm.org/D111883