libs: Return dummy location in Location::caller

[RyanGlScott]

Fixes #227.

[samcowger]

Out of curiosity, why not override Location::caller or intrinsics::caller_location in crucible-mir?

[RyanGlScott]

We could override intrinsics::caller_location in crucible-mir, I suppose. The main drawback of doing so is that it we'd have to fill out the fields of the Location struct using the crucible-mir API, which would be quite a bit more verbose than writing the same code directly in Rust. Doable, but perhaps a bit tiresome. Moreover, we'd need to ensure that we update this code any time the definition of Location changes.

Given this, I decided to go with the approach of patching Location::caller instead. This function is the only place in all of the standard libraries where intrinsics::caller_location is invoked, so patching Location::caller seemed to give the same benefits with less code. It's possible that there might be another crate in the wild that calls intrinsics::caller_location directly, but I haven't ran into one yet.