[clang] [clang][nfc] Guard thread_local in clang/test/CXX/drs/cwg7xx.cpp (PR #197930)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

We use `thread_local` in a number of tests in `tests/CXX` without checking for `__has_feature` or specifying target triple. How do they work on `arm64-apple-darwin-unknown`?

https://github.com/llvm/llvm-project/pull/197930