[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 We had to do a force push to ensure that the commit is associated with the right email address of @tigbr. He has two emails configured in github, and the UI somehow automatically picked the wrong one. The force push did not change any code.)

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