[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@328934 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed