[clang-tools-extra] [clang][dataflow] Change `diagnoseFunction` to take type of diagnostic list instead of diagnostic itself. (PR #66014)

[Yitzhak Mandelbaum via cfe-commits]

ymand wrote:

Updated to enforce `llvm::SmallVector`, per Martin's recommendation.  PTAL.

https://github.com/llvm/llvm-project/pull/66014