Pass sample pgo flags to thinlto.

Summary: ThinLTO needs to invoke SampleProfileLoader pass during link time in order to annotate profile correctly after module importing.

Reviewers: davidxl, mehdi_amini, tejohnson

Subscribers: pcc, davide, llvm-commits, mehdi_amini

Differential Revision: https://reviews.llvm.org/D27790

llvm-svn: 289957