Define DbiStreamBuilder::addSectionMap.

This change enables LLD to construct a Section Map stream in a PDB file.
I do not understand all these fields in the Section Map yet, but it seems
like a copy of a COFF section header in another format.

With this patch, DbiStreamBuilder can emit a Section Map which
llvm-pdbdump can dump.

Differential Revision: https://reviews.llvm.org/D26112

llvm-svn: 285606