[ThinLTO] Add per-module indexes to combined index consistently

Summary:
Without this change we only add module paths to the combined index when
there is a module hash or at least one global value. Make this more
consistent by adding the module to the index whenever there is a summary
section, and it is a per-module summary (had a MODULE_CODE_SOURCE_FILENAME
record).

Since we will no longer add module paths lazily, add a new interface to get
the module info from the index that asserts it is already added.

Fixes PR37899.

Reviewers: Vlad, pcc

Subscribers: mehdi_amini, inglorion, steven_wu, llvm-commits

Differential Revision: https://reviews.llvm.org/D48511

llvm-svn: 335567