[LTO] llvm-lto2: Add option to load sample profile

Summary:
This exposes LTO's Conf.SampleProfile as a command line option
(-lto-sample-profile-file) for testing via the llvm-lto2 utility.

Reviewers: pcc, danielcdh

Subscribers: mehdi_amini, inglorion, llvm-commits

Differential Revision: https://reviews.llvm.org/D36030

llvm-svn: 309456