[lldb] Let 'v' command directly access ivars of _any_ self/this

The `v` (`frame variable`) command can directly access ivars/fields of `this` or `self`.
Such as `v field`, instead of `v this->field`. This change relaxes the criteria for
finding `this`/`self` variables.

There are cases where a `this`/`self` variable does exist, but up to now the `v` command
has not made use of it. The user would have to explicitly run `v this->field` or
`self->_ivar` to access ivars. This change allows such cases to also work (without
explicitly dereferencing `this`/`self`).

A very common example in Objective-C (and Swift) is weakly capturing `self`:

```
__weak Type *weakSelf = self;
void (^block)(void) = ^{
   Type *self = weakSelf; // Re-establish strong reference.
   // `v _ivar` should work just as well as `v self->_ivar`.
};
```

In this case, `self` exists but `v` would not have used it. With this change, the fact
that a variable named `self` exists is enough for it to be used.

Differential Revision: https://reviews.llvm.org/D145276