[ELF] Refactor PltSection and IPltSection into PltSection [NFC]

    
Much of the code in PltSection and IPltSection is similar, we identify
the IPlt by a HeaderSize of 0 and alter our behaviour in the member
functions appropriately:
-Iplt does not have a header
-Iplt always follows after the Plt
    
Differential Revision: https://reviews.llvm.org/D29664

llvm-svn: 294579