[clang] Improve stack usage to increase recursive initialization depth (PR #88546)

[via cfe-commits]

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@@ -1163,7 +1137,7 @@ class Sema;
     ConversionSequenceList
     allocateConversionSequences(unsigned NumConversions) {
       ImplicitConversionSequence *Conversions =
-          slabAllocate<ImplicitConversionSequence>(NumConversions);
+          Ctx.Allocate<ImplicitConversionSequence>(NumConversions);
 
       // Construct the new objects.
       for (unsigned I = 0; I != NumConversions; ++I)
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cor3ntin wrote:

Maybe something like that would be cleaner

```cpp
 ConversionSequenceList allocateConversionSequences(unsigned NumConversions) {
      ImplicitConversionSequence *Conversions = Ctx.Allocate<ImplicitConversionSequence>(NumConversions);
      std::uninitialized_default_construct(Conversions, Conversions + NumConversions);
      return ConversionSequenceList(Conversions, NumConversions);
    }
```

https://github.com/llvm/llvm-project/pull/88546