[clang][dataflow] Change `diagnoseFunction` to use `llvm::SmallVector` instead...

[clang][dataflow] Change `diagnoseFunction` to use `llvm::SmallVector` instead of `std::vector`. (#66014)

The template is agnostic as to the type used by the list, as long as it
is
compatible with `llvm::move` and `std::back_inserter`. In practice,
we've
encountered analyses which use different types (`llvm::SmallVector` vs
`std::vector`), so it seems preferable to leave this open to the caller.