Revert "ASTReader: Copy input file offset data to avoid unaligned accesses"

We can do this better by changing the type to unaligned_uint64_t and
paying the cost on use instead of up front.

This reverts r240228

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@240246 91177308-0d34-0410-b5e6-96231b3b80d8