[pseudo] Eliminate the false `::` nested-name-specifier ambiguity

The solution is to favor the longest possible nest-name-specifier, and
drop other alternatives by using the guard, per per C++ [basic.lookup.qual.general].

Motivated cases:

```
Foo::Foo() {};
// the constructor can be parsed as:
//  - Foo ::Foo(); // where the first Foo is return-type, and ::Foo is the function declarator
//  + Foo::Foo(); // where Foo::Foo is the function declarator
```

```
void test() {

// a very slow parsing case when there are many qualifers!
X::Y::Z;
// The statement can be parsed as:
//  - X ::Y::Z; // ::Y::Z is the declarator
//  - X::Y ::Z; // ::Z is the declarator
//  + X::Y::Z;  // a declaration without declarator (X::Y::Z is decl-specifier-seq)
//  + X::Y::Z;  // a qualifed-id expression
}
```

Differential Revision: https://reviews.llvm.org/D130511

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