Recognize -fauto-profile in backend

[Diego Novillo]

This adds a new pass creator supports changing the name of the
input profile name. It's used by the -fauto-profile handler in
Clang:
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20131007/090356.html

Chandler, if it makes it easier to review, I could coalesce all
the separate patches I've been sending into a single one.


Thanks.  Diego.
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