Filled out platforms for Swift 5.4 availability.

Previously, the availability was "Future", meaning that any features
that were gated to be available in "Swift 5.4" would only be used if the
target were the highest possible version number (like iOS 99).  Here
that is fixed by using the OS versions that actually shipped with Swift
5.4.

rdar://75978438

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() {