[ELF] - Cleanup of LinkerScript<ELFT>::assignAddresses()

LinkerScript<ELFT>::assignAddresses is becoming larger and looks 
it can be good time for splitting. I expect to can more SectionsCommand's there, 
and dispatching some of them separatelly can help to keep method smaller either.

Differential revision: https://reviews.llvm.org/D22506

llvm-svn: 276300