Merge pull request swiftlang#36653 from nate-chandler/rdar75978438

Filled out platforms for Swift 5.4 availability.

Author: nate-chandler

lib/AST/Availability.cpp

@@ -393,7 +393,20 @@ AvailabilityContext ASTContext::getSwift53Availability() {
 }
 
 AvailabilityContext ASTContext::getSwift54Availability() {
-  return getSwiftFutureAvailability();
+  auto target = LangOpts.Target;
+
+  if (target.isMacOSX()) {
+    return AvailabilityContext(
+        VersionRange::allGTE(llvm::VersionTuple(11, 3, 0)));
+  } else if (target.isiOS()) {
+    return AvailabilityContext(
+        VersionRange::allGTE(llvm::VersionTuple(14, 5, 0)));
+  } else if (target.isWatchOS()) {
+    return AvailabilityContext(
+        VersionRange::allGTE(llvm::VersionTuple(7, 4, 0)));
+  } else {
+    return AvailabilityContext::alwaysAvailable();
+  }
 }
 
 AvailabilityContext ASTContext::getSwiftFutureAvailability() {