Add source level debugging info to LLVM output

[brson]

In addition to being useful for debugging, this seems to be a prerequisite for generating gcov output.

[msullivan]

That'll be really useful. Having rust line numbers in backtraces would be great.
How tricky would it be to be able to inspect variables? Is there a good way to describe our data types to a debugger?
This means we'll need to start tracking span information in trans properly, too.

[msullivan]

This looks to be a dupe of #237.