[clang] [clang][NFC] Reduce one of JSON dump tests (PR #195026)

[Vlad Serebrennikov via cfe-commits]

https://github.com/Endilll commented:

> A simpler way to put it, is that if JSON dumper has equal footing to the Text dumper, then it would only be reasonable that the each test case tests both equally.

Thank you for mentioning that, because I'd suggest this otherwise. PR is now updated with more matching of JSON dump, mirroring textual dump tests.

I left comments for two FIXMEs where I think JSON dump is missing information compared to textual dump. Fixing them is out of scope, but if @mizvekov can confirm whether they are real issues, I'd appreciate that.

https://github.com/llvm/llvm-project/pull/195026