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

/local/mnt/workspace/huihuiz/llvm-comm-git-2/llvm-project/llvm/lib/Object/MachOObjectFile.cpp:2731:7: warning: suggest braces around initialization of subobject [-Wmissing-braces]
      "i386",   "x86_64", "x86_64h",  "armv4t",  "arm",    "armv5e",
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      {
1 warning generated.

/local/mnt/workspace/huihuiz/llvm-comm-git-2/llvm-project/llvm/lib/Target/AMDGPU/AMDGPURegisterBankInfo.cpp:355:46: warning: suggest braces around initialization of subobject [-Wmissing-braces]
    return addMappingFromTable<1>(MI, MRI, { 0 }, Table);
                                             ^
                                             {}
1 warning generated.

/local/mnt/workspace/huihuiz/llvm-comm-git-2/llvm-project/llvm/tools/llvm-objcopy/ELF/Object.cpp:400:57: warning: suggest braces around initialization of subobject [-Wmissing-braces]
static constexpr std::array<uint8_t, 4> ZlibGnuMagic = {'Z', 'L', 'I', 'B'};
                                                        ^~~~~~~~~~~~~~~~~~
                                                        {                 }
1 warning generated.

llvm-svn: 372811