Summary:
1. we use the unsigned type for NextOffset,PrevOffset ,GlobalSymbolOffset , MemberTableSize, it will caused a malform big archive when the archive file size is large than 4G.
2. also fix a NFC comment on https://reviews.llvm.org/D142479#inline-1443927
Reviewers: James Henderson
Differential Revision: https://reviews.llvm.org/D150462