Handle UIDs and GIDs that don't fit in 6 decimal places.

Newer unix systems have 32 bit uid and gid types, but the archive format was
not updated. Fortunately, these fields are not normally used. Just truncate
the data to fit in 6 chars.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199223 91177308-0d34-0410-b5e6-96231b3b80d8