[LLDB] Run MSVC variant test with PDB (#171858)

Split off from #171489. This only adds the lookup of the active type for
a `std::variant` based on the head type (since PDB doesn't have template
info).

Author: Nerixyz

lldb/source/Plugins/Language/CPlusPlus/MsvcStlVariant.cpp

@@ -119,8 +119,16 @@ bool formatters::MsvcStlVariantSummaryProvider(
     storage_type = storage_type.GetTypedefedType();
 
   CompilerType active_type = storage_type.GetTypeTemplateArgument(1, true);
-  if (!active_type)
-    return false;
+  if (!active_type) {
+    // PDB: get the type from the head as we don't have template arguments
+    // there.
+    ValueObjectSP head = GetHead(*storage);
+    if (!head)
+      return false;
+    active_type = head->GetCompilerType();
+    if (!active_type)
+      return false;
+  }
 
   stream << " Active Type = " << active_type.GetDisplayTypeName() << " ";
   return true;

lldb/test/API/functionalities/data-formatter/data-formatter-stl/generic/variant/TestDataFormatterStdVariant.py

@@ -9,6 +9,8 @@
 
 
 class StdVariantDataFormatterTestCase(TestBase):
+    TEST_WITH_PDB_DEBUG_INFO = True
+
     def do_test(self):
         """Test that that file and class static variables display correctly."""