Cleanups 4: quoted()

[StephanTLavavej]

• Avoid repeating types in quoted() by using return {m,e,o,w};.
  • The types are verbose, and the constructors are implicit. I considered this slightly more convenient/readable than returning auto.
• Use char_traits::length() instead of a raw loop in quoted().
  • Please verify my claim that this does not change runtime behavior (we're getting the same answer, number of characters before the null terminator).
  • I think we might have thought that char_traits was unavailable, but it is totally available here, just through an epic-length chain of dependencies. Using char_traits is not only simpler, it has the potential to be faster for char (especially) and wchar_t.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.