[LLD/PDB] Emit first section contribution for DBI Module Descriptor.

Part of the DBI stream is a list of variable length structures
describing each module that contributes to the final executable.

One member of this structure is a section contribution entry that
describes the first section contribution in the output file for
the given module.

We have been leaving this structure unpopulated until now, so with
this patch it is now filled out correctly.

Differential Revision: https://reviews.llvm.org/D45832

llvm-svn: 330457