[MC] - Fix build bot.

Error was:
/home/buildslave/slave_as-bldslv8/lld-perf-testsuite/llvm/lib/MC/MCFragment.cpp:241:22: error: field 'Offset' will be initialized after field 'LayoutOrder' [-Werror,-Wreorder]
      Atom(nullptr), Offset(~UINT64_C(0)), LayoutOrder(0) {

http://lab.llvm.org:8011/builders/lld-perf-testsuite/builds/9628/steps/build-bin%2Flld/logs/stdio

llvm-svn: 348351