[torque]: Export methods for Torque @export-ed C++ classes

This fix is a bit of overkill, but ensures that verifier methods are
available across library boundaries.

Change-Id: I120560f8e8e0be0ffcba43b46197efca4db0a62c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2141739Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#67063}

src/torque/implementation-visitor.cc

@@ -4384,7 +4384,7 @@ void ImplementationVisitor::GenerateClassVerifiers(
 
     const char* verifier_class = "TorqueGeneratedClassVerifiers";
 
-    h_contents << "class " << verifier_class << "{\n";
+    h_contents << "class V8_EXPORT_PRIVATE " << verifier_class << "{\n";
     h_contents << " public:\n";
 
     for (const ClassType* type : TypeOracle::GetClasses()) {