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

[Andrei Golubev via llvm-commits]

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@@ -100,7 +100,18 @@ 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;
+    Size = static_cast<Size_T>(N);
+    assert(size() == N);
+  }
+
+  /// Set the array capacity to \p M, which the current array size must
+  /// not be greater than.
+  ///
+  /// This does not construct or destroy any elements in the vector.
+  void set_capacity(size_t M) {
+    assert(size() <= M);
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andrey-golubev wrote:

thanks, the assertion you propose would indeed be useful!

`size() <= M` is to just guarantee that we don't end up in a weird container state where `Size > Capacity`. but then using set_size() together with set_capacity() in the current sense would be problematic:

```
// capacity = 50, size = 40

// grow:
// set_size(70); // asserts: 70 <= capacity() fails
set_capacity(100); // OK
set_size(70); // OK

// shrink:
// set_capacity(10); // asserts: size() <= 10 fails
set_size(2); // OK
set_capacity(10); // OK
```

I guess due to this "swap" in the order, it might make it harder to write generic code? but then I again think of `Size > Capacity` problem.

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