[opt-transform-range] Do not assume that Range::iterator exists.

[gottesmm]

The reason not to do this is that some important range
adapters (e.x. llvm::iterator_range) do not have such a typedef. Rather than
changing upstream this commit just fixes the problem by inferring the iterator
type from the result of Range::begin().

TLDR: This makes OptionalTransformRange work with llvm::iterator_range.

[gottesmm]

@swift-ci smoke test

[atrick]

I spent a good hour trying to work around this yesterday. My approach was to move to llvm::map_range (in the next version of LLVM). Unfortunately, mapped_range has a bug because it doesn't propagate the return type correctly. So this is a good stop-gap.

[gottesmm]

@atrick once those improvements land in LLVM, we should start ripping out our own custom stuff.

[atrick]

@gottesmm Yeah, I have it on my TODO list to fix llvm::mapped_range so we can use it

[gottesmm]

@atrick we also should upstream something like optional transform range (that is something that maps -> optional and then filters).