[clang][dataflow] Add convenience function for analysing and `FunctionDecl` and diagnosing it.

The convenience function captures running the analysis and then collecting
diagnostics based on a `Diagnoser` object. This pattern is valuable to
clang-tidy checks which analyze a function at a time, though it could be more
generally useful for analysis clients.

Differential Revision: https://reviews.llvm.org/D156254