[CodingStandards] Clarify C++ Standard Library usage

The existing wording leaves it unclear if C++ standard library data
structures should be preferred over custom LLVM ones, e.g., SmallVector,
even though common practice seems clear on the issue. This change makes
the wording more explicit and aligns it better with the code base.

Some motivating statistics:

```
ag SmallVector llvm/lib/ | wc
  8846   40306  901421
 ag 'std::vector' llvm/lib/ | wc
  2123    8990  214482

ag SmallVector clang/lib/ | wc
  3023   13824  281691
ag 'std::vector' clang/lib/ | wc
   719    2914   72817
```

Differential Revision: https://reviews.llvm.org/D74340