Fix scoping of temporary variables in Darwin codegen. (#15123)

When converting a C++ DecodableList into an Objective C NSArray, we
declare some temporary variables.  These were not scoped in their own
scope, so if multiple lists were decoded at the same nesting level
(e.g. two list-typed members of the same struct), the names would
collide and produce compile errors.

The fix is to just scope the entire conversion.

src/darwin/Framework/CHIP/templates/partials/decode_value.zapt

@@ -11,21 +11,21 @@
     {{>decode_value target=target source=(concat source ".Value()") cluster=cluster errorCode=errorCode depth=(incrementDepth depth) isNullable=false}}
   }
 {{else if isArray}}
-  auto * array_{{depth}} = [NSMutableArray new];
-  auto iter_{{depth}} = {{source}}.begin();
-  while (iter_{{depth}}.Next()) {
-    auto & entry_{{depth}} = iter_{{depth}}.GetValue();
-    {{asObjectiveCClass type cluster forceNotList=true}} * newElement_{{depth}};
-    {{>decode_value target=(concat "newElement_" depth) source=(concat "entry_" depth) cluster=cluster errorCode=errorCode depth=(incrementDepth depth) isArray=false}}
-    [array_{{depth}} addObject:newElement_{{depth}}];
-  }
-  {  // Scope for the error so we will know what it's named
+  { // Scope for our temporary variables
+    auto * array_{{depth}} = [NSMutableArray new];
+    auto iter_{{depth}} = {{source}}.begin();
+    while (iter_{{depth}}.Next()) {
+      auto & entry_{{depth}} = iter_{{depth}}.GetValue();
+      {{asObjectiveCClass type cluster forceNotList=true}} * newElement_{{depth}};
+      {{>decode_value target=(concat "newElement_" depth) source=(concat "entry_" depth) cluster=cluster errorCode=errorCode depth=(incrementDepth depth) isArray=false}}
+      [array_{{depth}} addObject:newElement_{{depth}}];
+    }
     CHIP_ERROR err = iter_{{depth}}.GetStatus();
     if (err != CHIP_NO_ERROR) {
       {{errorCode}}
     }
+    {{target}} = array_{{depth}};
   }
-  {{target}} = array_{{depth}};
 {{else}}
   {{#if_is_struct type}}
     {{target}} = [{{asObjectiveCClass type cluster forceNotList=true}} new];