ADT: Make SmallVector::set_size() private

Stop allowing use of `SmallVectorBase::set_size()` outside of the
SmallVector implementation, which sets the size without calling
constructors or destructors.

Most callers should probably just use `resize()`. Or, if the new size is
guaranteed to be `<= size()`, then the new-ish `truncate()` works too
(and optimizes better).

Some callers want to avoid initializing memory before overwriting, but
need a pointer to the memory and so cannot use `push_back()`,
`emplace_back()`, or `append()`. Before this commit, this depended on
`reserve()` and `set_size()`:

```
V.reserve(V.size() + NumNew);      // Reserve expected size.
NumNew = initialize(V.end(), ...); // Get number added.
V.set_size(V.size() + NumNew);     // Set size to match.
```

Such code should be updated to use `resize_for_overwrite()` and
`truncate()`:

```
auto Size = V.size();                        // Save initial size.
V.resize_for_overwrite(Size + NumNew);       // Resize to expected size.
NumNew = initialize(V.begin() + Size, ...)); // Get number added.
V.truncate(Size + NumNew);                   // Truncate to match.
```

The new pattern is safe even for non-trivial types, since
`resize_for_overwrite()` calls constructors and `truncate()` calls
destructors. For trivial types, it should optimize the same way as the
old pattern.

Downstream code adapt to the disappearance of `set_size()` using this
new pattern should carefully audit uses of `V` between the resize and
the truncate:

- Change `V.size()` => `Size`.
- Change `V.capacity()` => `V.size()` (mostly).
- Change `V.end()` => `V.begin() + Size`.
- If `V` is an out-parameter, early returns need a `V.truncate()` or
  `V.clear()`. A scope exit is recommended.

Differential Revision: https://reviews.llvm.org/D115380

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