swift234

[Sema] Suppress deprecation/availability diagnostics in branches with empty availability

We currently emit diagnostics for references to deprecated and potentially
unavailable APIs in branches that are dead for the current platform and
minimum deployment target. These dead branches can arise when sharing swift
source between targets or in playgrounds intended to be run on multiple OS
versions.

So for example, suppose the developer has the following code targeting
OSX 10.9+ and iOS 8.0+:

if #available(OSX 10.10, *) {
  // Use replacement API introduced in OSX 10.10/iOS 8.0
} else {
  // Use old API deprecated in OSX 10.10/iOS 8.0
}

Compiling the above for iOS and with a miniminum deployment of 8.0 will
result in a deprecation diagnostic for the use of the API in the else branch
even though that branch will never execute. (Note that the branch is not dead on
the OSX target because the minimum deployment target is 10.9. For OSX we also
won't emit a deprecation warning because the API wasn't deprecated until 10.10.

This commit updates availability checking to create refinement contexts with
empty OS version ranges for #available() else branches when those branches
will definitely not execute. It suppresses deprecation and potential
availability diagnostics within those contexts. It does not suppress diagnostics
for references to APIs annotated as unconditionally unavailable (i.e., with
@available(OSX, unavailable, ...) and friends).

rdar://problem/22307360

Swift SVN r31631