Debugger: Fix #12940.

- Add new interface TeamFunctionSourceInformation. Currently this
exposes a single function allowing one to query for the currently
active source code given a FunctionDebugInfo instance.
- Implement TeamFunctionSourceInformation on TeamDebugInfo.
- Pass TeamFunctionSourceInformation to Dwarf{Team,Image}DebugInfo.
In turn, make use of it in DwarfImageDebugInfo::GetStatement() in
order to determine whether to return the corresponding assembly
or source statement.

With this piece of information, the debugger is now correctly able to
determine that the user is currently looking at disassembly despite debug
info being available, and consequently adjust its stepping behavior based on
that. Previously, the source code statement was always used, leading to it
not being possible to single step assembly lines in such a circumstance
without manually using run to cursor.

Other related cleanups:
- TeamDebugInfo now inherits BReferenceable directly, rather than relying on
indirectly inheriting it from TeamTypeInformation.
- Remove BReferenceable from TeamTypeInformation. The latter is only an
interface anyways, and inheriting that base class from multiple locations
was causing GCC5 trouble when resolving BReference<TeamDebugInfo>, even
when virtual inheritance was used.