[clang] [llvm] [ADT] Fix SmallVector assign iterator tag check (PR #191030)

[Alexis Engelke via cfe-commits]

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aengelke wrote:

@cor3ntin Ok, but what about this one? Inserting one-by-one would have terrible performance.

Also, should we then call distance only for random_access_iterators instead of iterating twice?

https://github.com/llvm/llvm-project/pull/191030