* rdar://105027055: Do not use as-casts in specializations.
Introduce _specializingCast and use it instead of `as?`-casts inside
if ladders that implement generic specialization.
`as?` invokes undesirable runtime machinery such as bridging checks.
* rdar://105027055: Ensure AttributedString views have properly aligned bounds
* rdar://105027055: Implement index(_:offsetBy:limitedBy:) in AttributedString views