[yaml2obj] - Change how we handle implicit sections.

[yaml2obj] - Change how we handle implicit sections.

Instead of having the special list of implicit sections,
that are mixed with the sections read from YAML on late
stages, I just create the placeholders and add them to
the main sections list early.

That allows to significantly simplify the code.

Differential revision: https://reviews.llvm.org/D64999

llvm-svn: 366677