Add IR support, ELF section and user documentation for partitioning feature.

Add IR support, ELF section and user documentation for partitioning feature.

The partitioning feature was proposed here:
http://lists.llvm.org/pipermail/llvm-dev/2019-February/130583.html

This is mostly just documentation. The feature itself will be contributed
in subsequent patches.

Differential Revision: https://reviews.llvm.org/D60242

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@361923 91177308-0d34-0410-b5e6-96231b3b80d8