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

In particular, it's important that we create the "fallback" atomic at
this point
(which we produce if the transfer function didn't produce a value for
the
expression) so that it is placed in the correct environment.

Previously, we processed the terminator condition in the
`TerminatorVisitor`,
which put the fallback atomic in a copy of the environment that is
produced as
input for the _successor_ block, rather than the environment for the
block
containing the expression for which we produce the fallback atomic.

As a result, we produce different fallback atomics every time we process
the
successor block, and hence we don't have a consistent representation of
the
terminator condition in the flow condition.

This patch includes a test (authored by ymand@) that fails without the
fix.