[ELF] Move createThunks() after scanRelocations()

    
A necessary first step towards range extension thunks is to delay
the creation of thunks until the layout of InputSections within
OutputSections has been done.
    
The change scans the relocations directly from InputSections rather
than looking in the ELF File the InputSection came from. This will
allow a future change to redirect the relocations to symbols defined
by Thunks rather than indirect when resolving relocations.
    
A side-effect of moving ThunkCreation is that the OutSecOff of
InputSections may change in an OutputSection that contains Thunks.
In well behaved programs thunks are not in OutputSections with
dynamic relocations.
    
Differential Revision: https://reviews.llvm.org/D28811

llvm-svn: 292359