[clang][dataflow] Fully support Environment construction for Stmt analysis. (#91616)

Assume in fewer places that the analysis is of a `FunctionDecl`, and
initialize the `Environment` properly for `Stmt`s.

Moves constructors for `Environment` to header to make it more obvious
that there are only minor differences between them and very little
initialization in the constructors.

Tested with check-clang-tooling.