Move accurate-sample-profile into the function attribute.

Summary: We need to have accurate-sample-profile in function attribute so that it works with LTO.

Reviewers: davidxl, rsmith

Reviewed By: davidxl

Subscribers: sanjoy, mehdi_amini, javed.absar, llvm-commits, eraman

Differential Revision: https://reviews.llvm.org/D37113

llvm-svn: 311706