[ELF] Set SectionBase::partition in processSectionCommands

Fixes PR43461 (regression caused by D67504)

The partition field of a SECTIONS-specified section is not set after
D67504. The 0 value affects findSection() which checks if the partition
field is 1.

So `Out::initArray = findSection(".init_array")` is null, and
DT_INIT_ARRAYSZ is not set.

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D68087

llvm-svn: 372996