Introduce a debug info builder.

This is my proposal for a new way to emit debug information. It is a pale copy of the `CGDebugInfo` class from clang. The whole implementation uses no global variables (which is my main motiviation). It also introduces a namespace `ldc`.

Possible enhancements are caches for DI types (again, see `CGDebugInfo`).

Next step would be to create a `LDCContext` and try to eliminate as much of `gIR` as possible.