[clang] [clang][bytecode] Fix crash on arrays with excessive size (PR #175402)

[Timm Baeder via cfe-commits]

================
@@ -424,7 +424,7 @@ Descriptor *Program::createDescriptor(const DeclTy &D, const Type *Ty,
         if (!ElemDesc)
           return nullptr;
         unsigned ElemSize = ElemDesc->getAllocSize() + sizeof(InlineDescriptor);
-        if (std::numeric_limits<unsigned>::max() / ElemSize <= NumElems)
+        if (Descriptor::MaxArrayElemBytes / ElemSize < NumElems)
----------------
tbaederr wrote:

This is not correct for composite elements. `MaxArrayElemBytes` includes the size of the `InitMapPtr`, so is only for primitive arrays.

Consider:
```
struct S { char c; };
S q[-2U];

void foo() { S *p = q + 1; }
```
This works without this change (and causes a the current interpreter to allocate too much memory).

https://github.com/llvm/llvm-project/pull/175402