Tweak to r250117 and change to use ErrorOr and drop isSizeValid for

ArchiveMemberHeader, suggestion by Rafael Espíndola.

Also The clang-x86-win2008-selfhost bot still does not like the
malformed-machos 00000031.a test, so removing it for now.  All
the other bots are fine with it however.

llvm-svn: 250222