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

[Andrew Browne via llvm-commits]

================
@@ -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);
----------------
browneee wrote:

Thanks.

After seeing this change, I think it would probably make sense to include this line in the new function:
```
this->BeginX = NewElts;
```
... setting the allocation pointer and the capacity together makes sense.

This would also change the name of the new function:
```
  /// Set the allocation to \p A and the capacity to \p M.
  ///
  /// This does not construct or destroy any elements in the vector.
  /// This does not clean up any existing allocation.
void set_allocation_and_capacity(void *A, size_t M) {
```

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