[stdlib] make _overrideLifetime() functions public

[glessard]

These functions are discussed in the lifetime annotations pitch: swiftlang/swift-evolution#2750
They are necessary to implement functions returning non-escapable values, such as Data.bytes.

Addresses rdar://150400414

[glessard]

@swift-ci please smoke test

[meg-gupta]

lgtm

[atrick]

Thanks! This is super helpful.

[atrick]

@glessard you can remove these TODOs now:

  // TODO: Remove @_unsafeNonescapableResult. Instead, the unsafe dependence
  // should be expressed by a builtin that is hidden within the function body.

They were originally to discourage people from using @_unsafeNonescapableResult in more places, but we won't be able to remove that annotation any time soon and the TODO is unhelpful for anyone not working on the compiler.