[LTO] Simplify APIs and constify (NFC)

Summary:
Multiple APIs were taking a StringMap for the ImportLists containing
the entries for for all the modules while operating on a single entry
for the current module. Instead we can pass the desired ModuleImport
directly. Also some of the APIs were not const, I believe just to be
able to use operator[] on the StringMap.

Reviewers: tejohnson

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D23537

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278776 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed