AST: Give all ValueDecls an interface type

Previously, getInterfaceType() would return getType() if no
interface type was set. Instead, always set an interface type
explicitly.

Eventually we want to remove getType() altogether, and this
brings us one step closer to this goal.

Note that ParamDecls are excempt from this treatment, because
they don't have a proper interface type yet. Cleaning this up
requires more effort.

lib/AST/Module.cpp

@@ -84,6 +84,7 @@ void BuiltinUnit::LookupCache::lookupValue(
                                           /*genericparams*/nullptr,
                                           const_cast<BuiltinUnit*>(&M));
       TAD->computeType();
+      TAD->setInterfaceType(TAD->getType());
       TAD->setAccessibility(Accessibility::Public);
       Entry = TAD;
     }
@@ -346,6 +347,7 @@ ModuleDecl::ModuleDecl(Identifier name, ASTContext &ctx)
   ctx.addDestructorCleanup(*this);
   setImplicit();
   setType(ModuleType::get(this));
+  setInterfaceType(ModuleType::get(this));
   setAccessibility(Accessibility::Public);
 }