[modules] Introduce new VariableLocation for module imports/exports.

src/full-codegen/x64/full-codegen-x64.cc

Index: src/full-codegen/x64/full-codegen-x64.cc
diff --git a/src/full-codegen/x64/full-codegen-x64.cc b/src/full-codegen/x64/full-codegen-x64.cc
index 612bd6a333570de59c38842a565155be6c8b235f..acc6a9e285d3a93956b6489c15827828bb3d9435 100644
--- a/src/full-codegen/x64/full-codegen-x64.cc
+++ b/src/full-codegen/x64/full-codegen-x64.cc
@@ -763,6 +763,9 @@ void FullCodeGenerator::VisitVariableDeclaration(
       PrepareForBailoutForId(proxy->id(), BailoutState::NO_REGISTERS);
       break;
     }
+
+    case VariableLocation::MODULE:
+      UNREACHABLE();
   }
 }
 
@@ -819,6 +822,9 @@ void FullCodeGenerator::VisitFunctionDeclaration(
       PrepareForBailoutForId(proxy->id(), BailoutState::NO_REGISTERS);
       break;
     }
+
+    case VariableLocation::MODULE:
+      UNREACHABLE();
   }
 }
 
@@ -1305,6 +1311,9 @@ void FullCodeGenerator::EmitVariableLoad(VariableProxy* proxy,
       context()->Plug(rax);
       break;
     }
+
+    case VariableLocation::MODULE:
+      UNREACHABLE();
   }
 }