[llvm] [LLVM][ADT] Explicitly convert size_t values to SmallVector's size type (PR #77939)

[Andrei Golubev via llvm-commits]

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@@ -468,7 +469,7 @@ void SmallVectorTemplateBase<T, TriviallyCopyable>::takeAllocationForGrow(
     free(this->begin());
 
   this->BeginX = NewElts;
-  this->Capacity = NewCapacity;
+  this->Capacity = static_cast<SmallVectorSizeType<T>>(NewCapacity);
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andrey-golubev wrote:

> > i guess there are also cases when we only want to set the capacity?
> 
> Such as?

Was thinking of realloc() but we'd anyway reset the pointer (since same memory block being "reused" is an implementation detail).

> I think setting a capacity is intrinsically linked to setting a new storage pointer, which is why it makes sense to do both at once..

yeah, fair point. I was wondering whether `data start != allocation start` but these seem to always be equal (I guess the case when they are not is the custom alignment requirements but I haven't found anything like this).

https://github.com/llvm/llvm-project/pull/77939