Add getHeader helper and move ToHeader to the cpp file.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185933 91177308-0d34-0410-b5e6-96231b3b80d8
We never compare iterators from two archives. Assert that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185934 91177308-0d34-0410-b5e6-96231b3b80d8
Archive members cannot be larger than 4GB. Return a uint32_t.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185936 91177308-0d34-0410-b5e6-96231b3b80d8
Add missing getters. They will be used in llvm-ar.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185937 91177308-0d34-0410-b5e6-96231b3b80d8