[clang] [clang][dataflow] Process terminator condition within `transferCFGBlock()`. (PR #78127)

[via cfe-commits]

martinboehme wrote:

This is a re-land of https://github.com/llvm/llvm-project/pull/77750, which I reverted; it caused build bots to fail because `TerminatorVisitor::StmtToEnv` became unused.

This version removes `TerminatorVisitor::StmtToEnv` entirely.

https://github.com/llvm/llvm-project/pull/78127