Line numbers in stack traces

[nikomatsakis]

We get function names currently but not line numbers.

[brson]

Related to #237

[jdm]

This was working at one point, but it doesn't seem to be any more. I have suspicions that the shift to using a bunch of libraries broke it, but I'm investigating.

[brson]

I saw line numbers in my stack traces today. It was awesome. That's all.

[jdm]

It's something to do with that LLVM patch I had you revert and the way directory prefixes are handled by LLVM and the debuginfo generator. There haven't been line numbers in any debug build of rustc that I have built.

[jdm]

Pretty sure that this is fixed with my latest round of debuginfo changes from last month.