Define DbiStreamBuilder::addSectionContribs.

This patch defines a new function to add a SectionContribs stream
to a PDB file. Unlike SectionMap, SectionContribs contains a list
of input sections as opposed to output sections.

Note that this patch needs improving because currently we do not
set Module field in SectionContribs entries. In a follow-up patch,
I'll add Modules and then fix it after that.

Differential Revision: https://reviews.llvm.org/D26210

llvm-svn: 286677