[clang] Fix crash with -ast-dump=json (PR #137324)

[Akira Hatanaka via cfe-commits]

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@@ -211,6 +211,8 @@ Non-comprehensive list of changes in this release
 - Added `__builtin_elementwise_exp10`.
 - For AMDPGU targets, added `__builtin_v_cvt_off_f32_i4` that maps to the `v_cvt_off_f32_i4` instruction.
 - Added `__builtin_elementwise_minnum` and `__builtin_elementwise_maxnum`.
+- No longer crashing on invalid Objective-C extensions when dumping the AST as
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ahatanak wrote:

I think "Invalid Objective-C categories and extensions" is better since both `@interface SomeClass (SomeExtension)` and `@interface SomeClass ()` cause clang to crash.

https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html

https://github.com/llvm/llvm-project/pull/137324