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

[Hans Wennborg via cfe-commits]

zmodem wrote:

Can we revert https://github.com/llvm/llvm-project/pull/197732 to unbreak the build while we discuss this? Or can we land my fix for now?

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