patch: add a driver flag to turn on function merging optimization

[Nick Lewycky]

The attached patch adds -fmerge-functions which is in turn attached to
PassManagerBuilder::MergeFunctions. This is provided for testing it out,
and we can decide what we want the default to be after we have experience.
Note that it does bad things to debug info which doesn't currently have a
way to represent which function got called if two functions were merged.

Please review!

Nick
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