Add debug info for boxes
Add debug info for vectors
Add debug info for bare_fn type
Prevent debug info generation of zero-span nodes

If a node has a (0, 0) span, it was not in the source, so debug symbols should not be generated for it.
Add debug info test for boxes
Add debug info test for vectors
xfail box and vec debug info tests
auto merge of #5824 : bleibig/rust/debuginfo, r=brson

This adds debugging symbol generation for boxes, bare functions, vectors, and strings, along with a tests for boxes and vectors.

Note that gdb will see them as their actual compiled representation with the refcount, tydesc, etc. fields, so if `b` refers to box, `b->boxed` will refer to its value. Also, since you seem to use the [C struct hack](http://c-faq.com/struct/structhack.html) for dynamic vectors, you won't be able to print out the whole vector at once, only one element at a time by indexing specific elements.