[llvm-objcopy] Fix latent bug that allowed some Sections to be improperly cast...

[llvm-objcopy] Fix latent bug that allowed some Sections to be improperly cast to StringTableSections

If a Section had Type SHT_STRTAB (which could happen if you had a
.dynstr section) it was possible to cast Section to StringTableSection
and get away with any operation that was supported by SectionBase
without it being noticed. This change makes this bug easier to notice
and fixes it where it occurred. It also made me realize that there was
some duplication of efforts in the loop that calls ::initialize. These
issues are all fixed by this change.

Differential Revision: https://reviews.llvm.org/D38329

llvm-svn: 315372