With the acceptance of SE-0458, allow the use of unsafe expressions, the
@safe and @unsafe attributes, and the `unsafe` effect on the for..in loop
in all Swift code.
Introduce the `-strict-memory-safety` flag detailed in the proposal to
enable strict memory safety checking. This enables a new class of
feature, an optional feature (that is *not* upcoming or experimental),
and which can be detected via `hasFeature(StrictMemorySafety)`.
Usage of Span was temporarily behind an experimental feature flag. Now
that SE-0447 has been accepted, remove the experimental feature flag and
allow Span usage everywhere.
Implements rdar://144819992.
