[ELF] Convert DynamicSection to input section.

This patch introduces the following changes:
- DynamicSection now inherits InputSection<ELFT> and was moved
  to SyntheticSections.h/.cpp.
- Link and Entsize fields of DynamicSection are propagated to 
  its output section
- In<ELFT>::SyntheticSections was removed.
- Finalization of synthetic sections was removed from 
  OutputSection<ELFT>::finalize. Now finalizeSyntheticSections is
  used instead.

Differential revision: https://reviews.llvm.org/D26603

llvm-svn: 286950