Add braces for CARBON_KIND uses that lack them (#5882)

Also document why these are expected on `CARBON_KIND`. In
`kind_switch_test.cpp`, drop the `str` variable.

My recollection of the original discussion of `CARBON_KIND` is that it
should always have braces due to the risk of confusion for statement
interpretation, similar to a typical `if`/`else` but more subtle due to
the macro.

For example:

```
      case CARBON_KIND(int n):
        str << "int = " << n;
        return str.TakeStr();
```

is equivalent to:

```
      case CARBON_KIND(int n): {
          str << "int = " << n;
        }
        return str.TakeStr();
```

This happens to work in context because `str` isn't scoped, but a
trivial refactoring to move `RawStringOstream str;` the first statement
of the `case` would probably have non-obvious results. For example:

```
      case CARBON_KIND(int n):
        RawStringOstream str; // Valid name shadowing, destructed without use.
        str << "int = " << n; // Name lookup error on `n`.
        return str.TakeStr();
```

toolchain/sem_ir/stringify.cpp

@@ -123,36 +123,46 @@ class StepStack {
   auto PushArray(llvm::ArrayRef<PushItem> items) -> void {
     for (auto item : llvm::reverse(items)) {
       CARBON_KIND_SWITCH(item) {
-        case CARBON_KIND(InstId inst_id):
+        case CARBON_KIND(InstId inst_id): {
           PushInstId(inst_id);
           break;
-        case CARBON_KIND(llvm::StringRef string):
+        }
+        case CARBON_KIND(llvm::StringRef string): {
           PushString(string);
           break;
-        case CARBON_KIND(NameId name_id):
+        }
+        case CARBON_KIND(NameId name_id): {
           PushNameId(name_id);
           break;
-        case CARBON_KIND(ElementIndex element_index):
+        }
+        case CARBON_KIND(ElementIndex element_index): {
           PushElementIndex(element_index);
           break;
-        case CARBON_KIND(QualifiedNameItem qualified_name):
+        }
+        case CARBON_KIND(QualifiedNameItem qualified_name): {
           PushQualifiedName(qualified_name.first, qualified_name.second);
           break;
-        case CARBON_KIND(EntityNameItem entity_name):
+        }
+        case CARBON_KIND(EntityNameItem entity_name): {
           PushEntityName(entity_name.first, entity_name.second);
           break;
-        case CARBON_KIND(EntityNameId entity_name_id):
+        }
+        case CARBON_KIND(EntityNameId entity_name_id): {
           PushEntityNameId(entity_name_id);
           break;
-        case CARBON_KIND(TypeId type_id):
+        }
+        case CARBON_KIND(TypeId type_id): {
           PushTypeId(type_id);
           break;
-        case CARBON_KIND(SpecificInterface specific_interface):
+        }
+        case CARBON_KIND(SpecificInterface specific_interface): {
           PushSpecificInterface(specific_interface);
           break;
-        case CARBON_KIND(llvm::ListSeparator * sep):
+        }
+        case CARBON_KIND(llvm::ListSeparator * sep): {
           PushString(*sep);
           break;
+        }
       }
     }
   }