[ADT] Introduce `map_to_vector` helper

The following pattern is common in the llvm codebase, as well as in downstream projects:
```
llvm::to_vector(llvm::map_range(container, lambda))
```
This patch introduces a shortcut for this called `map_to_vector`.

This template depends on both `llvm/ADT/SmallVector.h` and `llvm/ADT/STLExtras.h`, and since these are both relatively large and do not depend on each other, the `map_to_vector` helper is placed in a new header under `llvm/ADT/SmallVectorExtras.h`. Only a handful of use cases have been updated to use the new helper.

Differential Revision: https://reviews.llvm.org/D145390