Basic ODR checking for C++ modules:

If we have multiple definitions of the same entity from different modules, we
nominate the first definition which we see as being the canonical definition.
If we load a declaration from a different definition and we can't find a
corresponding declaration in the canonical definition, issue a diagnostic.

This is insufficient to prevent things from going horribly wrong in all cases
-- we might be in the middle of emitting IR for a function when we trigger some
deserialization and discover that it refers to an incoherent piece of the AST,
by which point it's probably too late to bail out -- but we'll at least produce
a diagnostic.