[NFC] Add { } to silence compiler warning [-Wmissing-braces].

llvm-project/llvm/unittests/ADT/ArrayRefTest.cpp:254:25: warning: suggest braces around initialization of subobject [-Wmissing-braces]
  std::array<int, 5> A1{42, -5, 0, 1000000, -1000000};
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
                        {                           }

llvm-svn: 372902