64-bit watchOS should imply a 5.3 minimum version of the Swift runtime

(cherry picked from commit b073df2)

Author: kubamracek

lib/AST/Availability.cpp

@@ -259,7 +259,7 @@ AvailabilityContext ASTContext::getSwift50Availability() {
     return AvailabilityContext(
                             VersionRange::allGTE(llvm::VersionTuple(12,2)));
   } else if (target.isWatchOS()) {
-    if (target.getArch() == llvm::Triple::ArchType::x86_64)
+    if (target.isArch64Bit())
       return AvailabilityContext::alwaysAvailable();
 
     return AvailabilityContext(
@@ -297,6 +297,9 @@ AvailabilityContext ASTContext::getSwift51Availability() {
     return AvailabilityContext(
                             VersionRange::allGTE(llvm::VersionTuple(13,0,0)));
   } else if (target.isWatchOS()) {
+    if (target.isArch64Bit())
+      return AvailabilityContext::alwaysAvailable();
+
     return AvailabilityContext(
                             VersionRange::allGTE(llvm::VersionTuple(6,0,0)));
   } else {

lib/Basic/Platform.cpp

@@ -417,14 +417,20 @@ swift::getSwiftRuntimeCompatibilityVersionForTarget(
       }
     }
   } else if (Triple.isWatchOS()) {
+    auto floorFor64bits = [&Triple](llvm::VersionTuple v) {
+      if (!Triple.isArch64Bit()) return v;
+      // 64-bit watchOS was introduced with Swift 5.3
+      return MAX(v, llvm::VersionTuple(5, 3));
+    };
+
     Triple.getWatchOSVersion(Major, Minor, Micro);
     if (Major <= 5) {
-      return llvm::VersionTuple(5, 0);
+      return floorFor64bits(llvm::VersionTuple(5, 0));
     } else if (Major <= 6) {
       if (Minor <= 1) {
-        return llvm::VersionTuple(5, 1);
+        return floorFor64bits(llvm::VersionTuple(5, 1));
       } else {
-        return llvm::VersionTuple(5, 2);
+        return floorFor64bits(llvm::VersionTuple(5, 2));
       }
     }
   }