SE-0376 (second review): Function back deployment

[xwu (Xiaodi Wu)]

@Ben_Cohen

PartialRangeUpTo never appears to users and is really an implementation detail of the range expression infrastructure – a mechanic of how we make the ..< prefix operator work. It is not a type to name other more visible parts of the language after.

We already have visible parts of the language that use this terminology:

• prefix(upTo:) and prefix(through:)
• stride(from:to:by:) and stride(from:through:by:)