[ThinLTO] Add option for printing import failure reasons

Summary:
Adds the option for the printing of summary information about functions
considered but rejected for importing during the thin link.

Reviewers: davidxl

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, llvm-commits

Differential Revision: https://reviews.llvm.org/D50881

llvm-svn: 340047