[lldb] Merge RangeArray and RangeVector

The two classes are equivalent, except:
- the former uses a llvm::SmallVector (with a configurable size), while
  the latter uses std::vector.
- the former has a typo in one of the functions name

This patch just leaves one class, using llvm::SmallVector, and defaults
the small size to zero. This is the same thing we did with the
RangeDataVector class in D56170.