Split the creation of program headers in a few steps.

IMHO this makes the code easier to read and should help with linker
scripts.

This is strongly based on D16575. The main differences are:

We record a range of sections, not every section in a program header.

scanHeaders takes case of deciding what goes in every program header,
including PT_GNU_RELRO

We create dummy sections for the start of the file

With this, program header creation has 3 isolated stages:

Map sections to program headers.
Assign addresses to *sections*
Looking at sections find the address and size of each program header.

Thanks to George Rimar for the initial version.

llvm-svn: 260453