[PGO] Fixed non-determinism with DenseSet storing function importing info.

Summary:
r296498 introduced a DenseSet to store function importing info.

Using this container causes a test failure in
test/Transform/SampleProfile/import.ll when in Reverse Iteration mode.

This patch orders IDs before iterating through this container.

Reviewers: danielcdh, mgrang

Reviewed By: danielcdh

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D37246

llvm-svn: 312012