[Swiftify] enable mutable span

[hnrklssn]

[Swiftify] Wrap if-expressions in Immediately Called Closures

When parameters in swiftified wrapper functions are nullable, we use
separate branches for the nil and nonnil cases, because
withUnsafeBufferPointer (and similar) cannot be called on nil.
If-expressions have some limitations on where they are allowed in the
grammar, and cannot be passed as arguments to a function. As such, when
the return value is also swiftified, we get an error when trying to
pass the if-expression to the UnsafeBufferPointer/Span constructor.
While it isn't pretty, the best way forward seems to be by wrapping the
if-expressions in Immediately Called Closures.

The closures have the side-effect of acting as a barrier for 'unsafe':
unsafe keywords outside the closure do not "reach" unsafe expressions
inside the closure. We therefore have to emit "unsafe" where unsafe
expressions are used, rather than just when returning.

rdar://148153063

[Swiftify] Emit Mutable[Raw]Span when possible

Previously wrappers would use UnsafeMutable[Raw]Pointer for mutable
pointers, and Span for non-const std::span, to prevent the compiler from
complaining that MutableSpan didn't exist.

Now that MutableSpan has landed we can finally emit MutableSpan without
causing compilation errors. While we had (disabled) support for MutableSpan
syntax already, some unexpected semantic errors required additional
changes:

• Mutable[Raw]Span parameters need to be inout (for mutation)
• inout ~Escapable paramters need explicit lifetime annotations
• MutableSpan cannot be directly bitcast to std::span, because it is
  ~Copyable, so they need unwrapping to UnsafeMutableBufferPointer

rdar://147883022

[hnrklssn]

@swift-ci please smoke test