[yaml2obj/obj2yaml] - Refine handling of SHT_GNU_verdef sections.

[yaml2obj/obj2yaml] - Refine handling of SHT_GNU_verdef sections.

This patch:
1) Makes `Version`, `Flags`, `VersionNdx` and `Hash` fields to be `Optional<>`.
2) Disallows dumping version definitions that have `vd_version != 1`.
   `vd_version` identifies the version of the structure itself.
   (https://refspecs.linuxfoundation.org/LSB_5.0.0/LSB-Core-generic/LSB-Core-generic/symversion.html,
    https://docs.oracle.com/cd/E19683-01/816-7777/chapter6-80869/index.html)
3) Stops dumping default values for `Version`, `Flags`, `VersionNdx` and `Hash` fields.
4) Refines testing.

Differential revision: https://reviews.llvm.org/D94659