[ThinLTO] Add an import cutoff for debugging/triaging

Summary:
Adds -import-cutoff=N which will stop importing during the thin link
after N imports. Default is -1 (no  limit).

Reviewers: wmi

Subscribers: inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D45127

llvm-svn: 328934