[llvm-ar] Flatten thin archives.

Summary:
Normal behavior for GNU ar is to flatten thin archives when adding them to another thin archive, i.e. add the members directly instead of nesting the archive.

Some refactoring done as part of this patch to ease things:
 - Consolidate `addMember`/`addLibMember` methods
 - Rename `addMember` to `addChildMember` to make it more visibly different at the call site that an archive child is passed instead of a regular member
 - Pass in a separate vector and splice it back into position instead of passing a vector + optional Pos (which makes expanding libs tricky)

This fixes PR37530 as raised by https://github.com/ClangBuiltLinux/linux/issues/279.

Reviewers: mstorsjo, pcc, ruiu

Reviewed By: mstorsjo

Subscribers: llvm-commits, tpimh, nickdesaulniers

Differential Revision: https://reviews.llvm.org/D56508

llvm-svn: 351120