ADT: Add SmallVectorImpl::truncate() to replace uses of set_size()

Add `SmallVectorImpl::truncate()`, a variant of `resize()` that cannot
increase the size.

- Compared to `resize()`, this has no code path for growing the
  allocation and can be better optimized.
- Compared to `set_size()`, this formally calls destructors, and does
  not skip any constructors.
- Compared to `pop_back_n()`, this takes the new desired size, which in
  many contexts is more intuitive than the number of elements to remove.

The immediate motivation is to pair this with `resize_for_overwrite()`
to remove uses of `set_size()`, which can then be made private.

Differential Revision: https://reviews.llvm.org/D115383