The LLVM backend can now run the behavior tests and standard library
tests, like the x86_64 backend can. This commit required me to make a
lot of changes to how the LLVM backend lowers debug information, and
while I was doing that, I improved a few things:

* `anyerror` is now an enum type (and other error sets just wrap it), so
  error values appear by name in debuggers

* Fixed broken lowering for tagged unions with zero-width payloads

* Associate container types with source locations in all cases

* Avoid depending on the order of type resolution (using the new
  `DebugConstPool` abstraction), so debug information will contain all
  available type information rather than just the subset which happens
  to be resolved when the backend lowers that debug type