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

[Duncan P. N. Exon Smith via llvm-commits]

dexonsmith wrote:

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

Looks like the `buildkit/github-pull-requests` check is still pending... if you ping me when it's complete, I'd be happy to merge!

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