Fix GCC handling of ATOMIC_VAR_INIT

r333325 from D47225 added warning checks, and the test was written to be C++11 correct by using ATOMIC_VAR_INIT (note that the committee fixed that recently...). It seems like GCC can't handle ATOMIC_VAR_INIT well because it generates 'type 'std::atomic<int>' cannot be initialized with an initializer list' on bot libcxx-libcxxabi-x86_64-linux-ubuntu-cxx03. Drop the ATOMIC_VAR_INITs since they weren't required to test the diagnostics.

llvm-svn: 333327