Add -fprofile-sample-use to Clang's driver.

Summary:
This adds a new option -fprofile-sample-use=filename to Clang. It tells
the driver to schedule the SampleProfileLoader pass and passes on the
name of the profile file to use.

Depends on D28

Test Plan: accepted, implementing here first for full on testing

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D29