Recommit r354328, r354329 "[obj2yaml][yaml2obj] - Add support of...

Recommit r354328, r354329 "[obj2yaml][yaml2obj] - Add support of parsing/dumping of the .gnu.version_r section."

Fix:
Replace
assert(!IO.getContext() && "The IO context is initialized already");
with
assert(IO.getContext() && "The IO context is not initialized");
(this was introduced in r354329, where I tried to quickfix the darwin BB
and seems copypasted the assert from the wrong place).

Original commit message:

The section is described here:
https://refspecs.linuxfoundation.org/LSB_1.3.0/gLSB/gLSB/symverrqmts.html

Patch just teaches obj2yaml/yaml2obj to dump and parse such sections.

We did the finalization of string tables very late,
and I had to move the logic to make it a bit earlier.
That was needed in this patch since .gnu.version_r adds strings to .dynstr.
This might also be useful for implementing other special sections.

Everything else changed in this patch seems to be straightforward.

Differential revision: https://reviews.llvm.org/D58119

llvm-svn: 354335