Add { } to silence compiler warning [NFC]

At least clang 3.6 warns on the original code:

../tools/clang/lib/CodeGen/CGNonTrivialStruct.cpp:829:34: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
  return std::array<Address, 1>({Address(nullptr, CharUnits::Zero())});
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                 {                                  }
../tools/clang/lib/CodeGen/CGNonTrivialStruct.cpp:833:34: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
  return std::array<Address, 2>({Address(nullptr, CharUnits::Zero()),
                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2 errors generated.

llvm-svn: 358152