[ELF] - Split struct In<ELFT> to have templated and non-templated sections areas.

Patch splits In<ELFT> into 2 classes: one for non-templated sections, 
second contains ELFT templated ones.
That allows to code that was detemplated to access non-templated sections freely,
and should open road for futher detemplation proccess.

Differential revision: https://reviews.llvm.org/D30939

llvm-svn: 297844