[NFC] AST: Add always available availability ctxt.

Author: nate-chandler

include/swift/AST/AvailabilityContext.h

@@ -75,6 +75,9 @@ class AvailabilityContext {
   /// of the given declaration.
   static AvailabilityContext forDeclSignature(const Decl *decl);
 
+  /// Returns the unconstrained availability context.
+  static AvailabilityContext forAlwaysAvailable(const ASTContext &ctx);
+
   /// Returns the range of platform versions which may execute code in the
   /// availability context, starting at its introduction version.
   // FIXME: [availability] Remove; superseded by getAvailableRange().

lib/AST/AvailabilityContext.cpp

@@ -214,6 +214,13 @@ AvailabilityContext AvailabilityContext::forDeclSignature(const Decl *decl) {
   return forLocation(decl->getLoc(), decl->getInnermostDeclContext());
 }
 
+AvailabilityContext
+AvailabilityContext::forAlwaysAvailable(const ASTContext &ctx) {
+  return AvailabilityContext(Storage::get(AvailabilityRange::alwaysAvailable(),
+                                          /*isDeprecated=*/false,
+                                          /*domainInfos=*/{}, ctx));
+}
+
 AvailabilityRange AvailabilityContext::getPlatformRange() const {
   return storage->platformRange;
 }