[clang] [clang][dataflow] Re-land: Retrieve members from accessors called usi… (PR #74336)

[Samira Bazuzi via cfe-commits]

bazuzi wrote:

@martinboehme or @ymand Could you take a look and merge?

The only modification from https://github.com/llvm/llvm-project/pull/73978 is the `Env.initialize()` in the test.

https://github.com/llvm/llvm-project/pull/74336