Only seek once before writing the member offsets.
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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@224025 91177308-0d34-0410-b5e6-96231b3b80d8

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