Only seek once before writing the member offsets.

This cuts down the number on system calls done by a static llvm-ar producing
lib/libclangSema.a from 9164 to 442.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224025 91177308-0d34-0410-b5e6-96231b3b80d8