[mlir][ODS] Fix `VariadicRegion` code generation for `NoTerminator` Ops

[vinograd47]

Summary

The issue was introduced in D98468.

The {0}Regions is an array of std::unique_ptr<Region> objects,
so it should be processed accordingly.

[mehdi_amini]

Is it possible to make it exercised in tree? Maybe in the TestDialect?

[mehdi_amini]

(if this unblocks you, feel free to land it and send me a test afterward)

[vinograd47]

Added a test for the issue

mlir/tools/mlir-tblgen/OpFormatGen.cpp

for (auto &region : {0}Regions)
     ensureTerminator(*region, parser.getBuilder(), result.location);
 )";
 
 /// The code snippet used to ensure a list of regions have a block.
 ///
 /// {0}: The name of the region list.
 const char *regionListEnsureSingleBlockParserCode = R"(
   for (auto &region : {0}Regions)
-    if (region.empty()) *{0}Region.emplaceBlock();
+    if (region->empty()) region->emplaceBlock();
 )";
 
 /// The code snippet used to generate a parser call for an optional region.
 ///
 /// {0}: The name of the region.
 const char *optionalRegionParserCode = R"(
   {
      auto parseResult = parser.parseOptionalRegion(*{0}Region);