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

[Andrew Browne via llvm-commits]

================
@@ -99,8 +99,9 @@ template <class Size_T> class SmallVectorBase {
   ///
   /// This does not construct or destroy any elements in the vector.
   void set_size(size_t N) {
-    assert(N <= capacity());
-    Size = N;
+    auto n = static_cast<Size_T>(N);
+    assert(n <= capacity());
----------------
browneee wrote:

`capacity()` also returns `size_t`, so probably better to keep the assertion first. e.g.

```
    assert(N <= capacity());
    Size = static_cast<Size_T>(N);

    // As Felipe suggests, you could add:
    assert(static_cast<size_t>(Size) == N);
```

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