Use platform versions in getSwift57Availability.

Author: nate-chandler

lib/AST/Availability.cpp

@@ -512,7 +512,20 @@ AvailabilityContext ASTContext::getSwift56Availability() {
 }
 
 AvailabilityContext ASTContext::getSwift57Availability() {
-  return getSwiftFutureAvailability();
+  auto target = LangOpts.Target;
+
+  if (target.isMacOSX()) {
+    return AvailabilityContext(
+        VersionRange::allGTE(llvm::VersionTuple(13, 0, 0)));
+  } else if (target.isiOS()) {
+    return AvailabilityContext(
+        VersionRange::allGTE(llvm::VersionTuple(16, 0, 0)));
+  } else if (target.isWatchOS()) {
+    return AvailabilityContext(
+        VersionRange::allGTE(llvm::VersionTuple(9, 0, 0)));
+  } else {
+    return AvailabilityContext::alwaysAvailable();
+  }
 }
 
 AvailabilityContext ASTContext::getSwiftFutureAvailability() {