[stdlib] Replace `.Some(x)` and `.None` by `x` and `nil`, respectively

The standard library already predominantly uses the latter form but still
contains regular usages of the former. For consistency and reduced clutter
I'd propose to use the latter form throughout. The SIL for both forms is
identical. Also Optional<T> takes precedence over any other nil literal
convertibles, so no issues there. Are there any specific reason to use the
explicit forms?

Filed an issue (SR-212 <https://bugs.swift.org/browse/SR-212&gt;\) and pull
request (#470 <https://github.com/apple/swift/pull/470&gt;\).