[llvm-readobj][yaml2obj] Support SHT_LLVM_SYMPART, SHT_LLVM_PART_EHDR and SHT_LLVM_PART_PHDR

See http://lists.llvm.org/pipermail/llvm-dev/2019-February/130583.html
and D60242 for the lld partition feature.

This patch:

* Teaches yaml2obj to parse the 3 section types.
* Teaches llvm-readobj/llvm-readelf to dump the 3 section types.

There is no test for SHT_LLVM_DEPENDENT_LIBRARIES in llvm-readobj. Add
it as well.

Reviewed By: thakis

Differential Revision: https://reviews.llvm.org/D67228

llvm-svn: 371157