[ThinLTO] Fix printing of module paths for distributed backend indexes

Summary:
In the individual index files emitted for distributed ThinLTO backends,
the module path ids are not contiguous. Assign slots to module paths in
order to handle this better and also to get contiguous numbering in the
summary assembly.

Reviewers: davidxl, dexonsmith

Subscribers: mehdi_amini, inglorion, eraman, llvm-commits, steven_wu

Differential Revision: https://reviews.llvm.org/D48698

llvm-svn: 336148