IR: Tweak the API around adding modules to the summary index.

The current name (addModulePath) and return value
(ModulePathStringTableTy::iterator) is a little confusing. This
API adds a module, not just a path. And the iterator is basically
just an implementation detail of the summary index. Address
both of those issues by renaming to addModule and introducing a
ModuleSummaryIndex::ModuleInfo type that the function returns.

Differential Revision: https://reviews.llvm.org/D34124

llvm-svn: 305422