[PATCH] Generate cold attribute for functions marked __atribute__((cold))

This removes a FIXME in CodeGenModule::SetLLVMFunctionAttributesForDefinition.
When a function is declared cold we can now generate the IR attribute in
addition to marking the function to be optimized for size.

I tried adding a separate CHECK in the existing test, but it was
failing.  I suppose CHECK matches one line exactly once?  This would be
a problem if the attributes are listed in a different order, though they
seem to be sorted.