Fix exhaustive switches 29/x

Summary:
https://www.internalfb.com/wiki/IOS_at_Meta/switch_statements_in_C_languages/

*Exhaustive Switches*, i.e. using `-Wswitch-enum` and `-Wswitch-default`, are the ideal default for all C language code at Meta.

This diff is part of the migration of *Instagram* to adopting *Exhaustive Switches* by default.  Currently only `-Wswitch-enum` is enabled for *Instagram* apps, but we want to add `-Wswitch-default` to prevent bugs from undefined/unpredicted behavior (which means adding these `default:` cases will NOT interfere with the compiler putting up an error when a new value is added to an enum, like we want).

We will follow the recommended solutions as outlined in https://www.internalfb.com/wiki/Apple_Build_Errors/Missing_switch_default_Label/

  1. **Fail Fast**: [the vast majority of cases] use an `abort()` to forcibly crash the app on unexpected values.  Always use this when the existing behavior would be unpredictable/undefined if you were to introduce `default: break;`.  Also use this if one of the other options do not fit.
  2. **Handle unexpected values (A)**: when there is minimal code after the `switch` that only the unhandled values would reach, pull it into the `default:`
  3. **Handle unexpected values (B)**: when the code after the `switch` is safe for unhandled values but also needed for handled values, add a `default: break;` statement (potentially with an `FBCReportMustFix(...)`).
  4. **Handle unexpected values (C)**: when the enum type is from the platform itself (not our monorepo), and is an `NS_ENUM` type, we can make a judgement call if think the enum might expand and thus add a default handling behavior.  If there is a reasonable chance the platform could expand the enum in a future OS release, we can avoid crashing until we start building against the new SDK where the `-Wswitch-enum` will help us explicitly handle the new case.
  5. **Convert to *Flexible Switch* or to `if` checks**: on the rare occasion, it can be evident that we do not need an *exhaustive* switch and can use `FB_FLEXIBLE_SWITCH_BEGIN|END` or just use `if` checks.

Reviewed By: aary

Differential Revision: D76546644

fbshipit-source-id: 7c1fe1112bc4de10f83e97c1c9981d39c369761e

Author: NSProgrammer

third-party/folly/src/folly/json/dynamic-inl.h

@@ -1359,6 +1359,8 @@ class FormatValue<dynamic> {
       case dynamic::OBJECT:
         FormatValue(val_.at(arg.splitKey().toString())).format(arg, cb);
         break;
+      default:
+        folly::assume_unreachable();
     }
   }