Add OpenBSD to PlatformKinds.def.

In #31686 changes were introduced to ensure that capacity was stored in
the ManagedBuffer allocation, and @lorentey sugested that as a stopgap
measure for addressing the lack of platform malloc introspection on
OpenBSD, we use Swift availability attributes instead on the relevant
parts of ManagedBuffer and friends.

Since platform availability symbols must be specifically set up to be
used, this commit does so in advance of the above change.

include/swift/AST/PlatformKinds.def

@@ -32,5 +32,6 @@ AVAILABILITY_PLATFORM(watchOSApplicationExtension, "application extensions for w
 AVAILABILITY_PLATFORM(macOSApplicationExtension, "application extensions for macOS")
 AVAILABILITY_PLATFORM(macCatalyst, "Mac Catalyst")
 AVAILABILITY_PLATFORM(macCatalystApplicationExtension, "application extensions for Mac Catalyst")
+AVAILABILITY_PLATFORM(OpenBSD, "OpenBSD")
 
 #undef AVAILABILITY_PLATFORM

lib/AST/PlatformKind.cpp

@@ -87,6 +87,8 @@ static bool isPlatformActiveForTarget(PlatformKind Platform,
     case PlatformKind::watchOS:
     case PlatformKind::watchOSApplicationExtension:
       return Target.isWatchOS();
+    case PlatformKind::OpenBSD:
+      return Target.isOSOpenBSD();
     case PlatformKind::none:
       llvm_unreachable("handled above");
   }

lib/ClangImporter/ClangImporter.cpp

@@ -1930,6 +1930,10 @@ PlatformAvailability::PlatformAvailability(const LangOptions &langOpts)
         "APIs deprecated as of macOS 10.9 and earlier are unavailable in Swift";
     break;
 
+  case PlatformKind::OpenBSD:
+    deprecatedAsUnavailableMessage = "";
+    break;
+
   case PlatformKind::none:
     break;
   }
@@ -1962,6 +1966,9 @@ bool PlatformAvailability::isPlatformRelevant(StringRef name) const {
   case PlatformKind::watchOSApplicationExtension:
     return name == "watchos" || name == "watchos_app_extension";
 
+  case PlatformKind::OpenBSD:
+    return name == "openbsd";
+
   case PlatformKind::none:
     return false;
   }
@@ -2001,6 +2008,10 @@ bool PlatformAvailability::treatDeprecatedAsUnavailable(
   case PlatformKind::watchOSApplicationExtension:
     // No deprecation filter on watchOS
     return false;
+
+  case PlatformKind::OpenBSD:
+    // No deprecation filter on OpenBSD
+    return false;
   }
 
   llvm_unreachable("Unexpected platform");

lib/PrintAsObjC/DeclAndTypePrinter.cpp

@@ -852,6 +852,9 @@ private:
       case PlatformKind::watchOSApplicationExtension:
         plat = "watchos_app_extension";
         break;
+      case PlatformKind::OpenBSD:
+        plat = "openbsd";
+        break;
       case PlatformKind::none:
         llvm_unreachable("handled above");
       }

lib/SymbolGraphGen/AvailabilityMixin.cpp

@@ -56,6 +56,8 @@ StringRef getDomain(const AvailableAttr &AvAttr) {
       return { "tvOSAppExtension" };
     case swift::PlatformKind::watchOSApplicationExtension:
       return { "watchOSAppExtension" };
+    case swift::PlatformKind::OpenBSD:
+      return { "OpenBSD" };
     case swift::PlatformKind::none:
       return { "*" };
   }

lib/TBDGen/TBDGen.cpp

@@ -249,6 +249,8 @@ getLinkerPlatformId(OriginallyDefinedInAttr::ActiveVersion Ver) {
   switch(Ver.Platform) {
   case swift::PlatformKind::none:
     llvm_unreachable("cannot find platform kind");
+  case swift::PlatformKind::OpenBSD:
+    llvm_unreachable("not used for this platform");
   case swift::PlatformKind::iOS:
   case swift::PlatformKind::iOSApplicationExtension:
     return Ver.IsSimulator ? LinkerPlatformId::iOS_sim:

test/IDE/complete_decl_attribute.swift

@@ -39,6 +39,7 @@ struct MyStruct {}
 // AVAILABILITY1-NEXT: Keyword/None:                       macOSApplicationExtension[#Platform#]; name=macOSApplicationExtension{{$}}
 // AVAILABILITY1-NEXT: Keyword/None:                       macCatalyst[#Platform#]; name=macCatalyst
 // AVAILABILITY1-NEXT: Keyword/None:                       macCatalystApplicationExtension[#Platform#]; name=macCatalystApplicationExtension
+// AVAILABILITY1-NEXT: Keyword/None:                       OpenBSD[#Platform#]; name=OpenBSD{{$}}
 // AVAILABILITY1-NEXT: End completions
 
 @available(*, #^AVAILABILITY2^#)

tools/SourceKit/lib/SwiftLang/SwiftDocSupport.cpp

@@ -674,6 +674,7 @@ static void reportAttributes(ASTContext &Ctx,
   static UIdent PlatformOSXAppExt("source.availability.platform.osx_app_extension");
   static UIdent PlatformtvOSAppExt("source.availability.platform.tvos_app_extension");
   static UIdent PlatformWatchOSAppExt("source.availability.platform.watchos_app_extension");
+  static UIdent PlatformOpenBSD("source.availability.platform.openbsd");
   std::vector<const DeclAttribute*> Scratch;
 
   for (auto Attr : getDeclAttributes(D, Scratch)) {
@@ -702,6 +703,8 @@ static void reportAttributes(ASTContext &Ctx,
         PlatformUID = PlatformtvOSAppExt; break;
       case PlatformKind::watchOSApplicationExtension:
         PlatformUID = PlatformWatchOSAppExt; break;
+      case PlatformKind::OpenBSD:
+        PlatformUID = PlatformOpenBSD; break;
       }
 
       AvailableAttrInfo Info;