ELF: Simplify program header iteration

Instead of GetProgramHeaderCount+GetProgramHeaderByIndex, expose an
ArrayRef of all program headers, to enable range-based iteration.
Instead of GetSegmentDataByIndex, expose GetSegmentData, taking a
program header (reference).

This makes the code simpler by enabling range-based loops and also
allowed to remove some null checks, as it became locally obvious that
some pointers can never be null.

llvm-svn: 348928