swift234

Sema: Emit Fix-Its for potential unavailability

We now suggest up to three Fix-Its for each reference to a potentially
unavailable symbol: one to wrap the reference in an if #os(...) { ... }
guard (if possible), one to add an @availability attribute to an enclosing
property or method (if possible), and one to add an @availability attribute
to an enclosing class/struct/extension, etc. or global function.

The goal here is not to infer the "best" Fix-It but rather to ensure
discoverability of #os() and @availability attributes. We want the user, when
faced with an availability diagnostic, to be aware of the tools in her toolbox
to deal with it.

This is still missing QoI improvements, including Fix-Its to update
existing @availability attributes and more precise wording in diagnostics
(e.g, "initializer" instead of function, "class" instead of "type"). These
improvements will come in later commits.

Swift SVN r26073