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

[Andrei Golubev via llvm-commits]

andrey-golubev wrote:

CC @dexonsmith @alinas @nikic @browneee you seem to be working on / around SmallVector! feel free to add other reviewers as you see fit.

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