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

[Andrei Golubev via llvm-commits]

andrey-golubev wrote:

@dexonsmith added the comment but also squashed the changes for further merge. also, could you click the merge (if you think the changes are correct)? I lack merge access.

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