[Type checker] Get all conformances for externally-defined types.

IR generation depends on these when it emits type metadata.

Author: DougGregor

lib/Sema/TypeChecker.cpp

@@ -541,8 +541,10 @@ static void typeCheckFunctionsAndExternalDecls(SourceFile &SF, TypeChecker &TC)
         TC.checkFunctionErrorHandling(AFD);
         continue;
       }
-      if (isa<NominalTypeDecl>(decl))
+      if (auto nominal = dyn_cast<NominalTypeDecl>(decl)) {
+        (void)nominal->getAllConformances();
         continue;
+      }
       if (isa<VarDecl>(decl))
         continue;
       llvm_unreachable("Unhandled external definition kind");