[thinlto] Don't decay threshold for hot callsites

Summary:
We don't want to decay hot callsites to import chains of hot
callsites. The same mechanism is used in LIPO.

Reviewers: tejohnson, eraman, mehdi_amini

Subscribers: llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D24976

llvm-svn: 282833