[llvm-objcopy] New layout algorithm that lays out segments first

The current file layout algorithm in llvm-objcopy is simple but
difficult to reason about. It also makes it very complicated to support
nested segments and to support segments that have offsets that come
before a point after the program headers. To support these cases and
simplify one of the most critical parts llvm-objcopy I rewrote the
layout algorithm. Laying out segments first solves most of the issues
encountered by the previous algorithm.

Patch by Jake Ehrlich

Differential Revision: https://reviews.llvm.org/D36494

llvm-svn: 311825