[clang] [Clang] Give empty template parameter mapping an empty MLTAL (PR #164051)

[Corentin Jabot via cfe-commits]

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@@ -2017,8 +2022,13 @@ void SubstituteParameterMappings::buildParameterMapping(
       SemaRef.MarkUsedTemplateParameters(Args->arguments(),
                                          /*Depth=*/0, OccurringIndices);
   }
+  unsigned Size = OccurringIndices.count();
+  // It's OK when Size is 0. We build an empty parameter mapping when the atomic
+  // constraint is independent of any template parameters, so we can distinguish
+  // it from cases where no mapping exists at all, e.g. when there are only
+  // atomic constraints.
----------------
cor3ntin wrote:

```suggestion
  // When the constraint is independent of any template parameters, 
  // we build an empty mapping so that we can distinguish these cases 
  // from cases where no mapping exists at all, e.g. when there are only atomic constraints.
```

https://github.com/llvm/llvm-project/pull/164051