Simplify writeArchive return type.

Simplify writeArchive return type.

writeArchive returned a pair, but the first element of the pair is always
its first argument on failure, so it doesn't make sense to return it from
the function. This patch change the return type so that it does't return it.

Differential Revision: https://reviews.llvm.org/D37313

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@312177 91177308-0d34-0410-b5e6-96231b3b80d8