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

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> Yes, for example the OpenMP generated tests are a frequent source of frustration when changing dumping behavior.

I can confirm that OpenMP tests were a significant pain for me, but the test I'm changing here is in an entirely different league in terms of both size and signal-to-noise ratio.

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