[Frontend/Serialization] Remove ExtensibleEnums experimental flag

For now the semantics provided by `@extensible` keyword on per-enum
basis. We might return this as an upcoming feature in the future with
a way to opt-out.

(cherry picked from commit bf19481ab6)

lib/Serialization/ModuleFileSharedCore.cpp

@@ -225,9 +225,6 @@ static bool readOptionsBlock(llvm::BitstreamCursor &cursor,
     case options_block::STRICT_MEMORY_SAFETY:
       extendedInfo.setStrictMemorySafety(true);
       break;
-    case options_block::EXTENSIBLE_ENUMS:
-      extendedInfo.setSupportsExtensibleEnums(true);
-      break;
     default:
       // Unknown options record, possibly for use by a future version of the
       // module format.
@@ -1507,7 +1504,6 @@ ModuleFileSharedCore::ModuleFileSharedCore(
       Bits.AllowNonResilientAccess = extInfo.allowNonResilientAccess();
       Bits.SerializePackageEnabled = extInfo.serializePackageEnabled();
       Bits.StrictMemorySafety = extInfo.strictMemorySafety();
-      Bits.SupportsExtensibleEnums = extInfo.supportsExtensibleEnums();
       MiscVersion = info.miscVersion;
       SDKVersion = info.sdkVersion;
       ModuleABIName = extInfo.getModuleABIName();