[clang] [NFC][analyzer] Refactor Environment to map Expr to SVal instead of Stmt to SVal (PR #193295)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

@steakhal My analysis run verified that this change is indeed NFC, it did not change the set of results and did not introduce any crash.

https://github.com/llvm/llvm-project/pull/193295