Fix SectionPiece size when compiling with MSVC

Builds were failing with:

  InputSection.h(139): error C2338: SectionPiece is too big

because MSVC does record layout differently, probably not packing the
'OutputOff' and 'Live' bitfields because their types are of different
size. Using size_t for 'Live' seems to fix it.

llvm-svn: 284740