[clang][dataflow] Extend debug output for `Environment`. (#79982)

*  Print `ReturnLoc`, `ReturnVal`, and `ThisPointeeLoc` if applicable.

* For entries in `LocToVal` that correspond to declarations, print the
names
   of the declarations next to them.

I've removed the FIXME because all relevant fields are now being dumped.
I'm
not sure we actually need the capability for the caller to specify which
fields
to dump, so I've simply deleted this part of the comment.

Some examples of the output:


![image](https://github.com/llvm/llvm-project/assets/29098113/17d0978f-b86d-4555-8a61-d1f2021f8d59)


![image](https://github.com/llvm/llvm-project/assets/29098113/021dbb24-5fe2-4720-8a08-f48dcf4b88f8)