[flow analysis] Re-work handling of CFE null-aware guards.

The CFE desugars null-aware expressions such as `a?.b` into equivalent
`let` expressions involving a temporary guard variable (e.g., `let x =
a in x == null ? null : x.b`). This desugaring happens in the body
builder, prior to flow analysis. As a result, the expressions and
variables passed to flow analysis are those of the desugared `let`
expression rather than those of the code the user wrote. So a hack is
needed to ensure that flow analysis produces the correct result; the
hack involves promoting the temporary guard variable in the code path
where it is not null.

Prior to this change, the hack was done entirely in the CFE. Instead
of making a single pair of calls to
`FlowAnalysis.nullAwareAccess_rightBegin` and
`FlowAnalysis.nullAwareAccess_end` for every null-aware expression,
the CFE made two nested pairs of calls: one to promote the true target
of the null-aware access, and the other to promote the temporary guard
variable.

This had the advantage of keeping the hack entirely in the CFE, but
unfortunately it got in the way of fixing
https://github.com/dart-lang/language/issues/4344, because it
prevented flow analysis from properly recognizing field accesses
inside null-aware expressions.

This change adds an optional `guardVariable` parameter to
`FlowAnalysis_nullAwareAccess_rightBegin`, and shifts the
responsibility for promoting the temporary guard variable to the flow
analysis engine itself. With this change, the CFE no longer needs to
make two nested pairs of calls to
`FlowAnalysis.nullAwareAccess_rightBegin` and
`FlowAnalysis.nullAwareAccess_end`, and flow analysis now has the
necessary information to recognize field accesses inside null-aware
expressions.

I will perform the actual fix for
https://github.com/dart-lang/language/issues/4344 in a follow-up CL.

Note that in the future, I would like to change the CFE so that it
desugars null-aware accesses during type inference rather than during
the body builder; this will allow the expressions and variables passed
to flow analysis to be precisely those of the code the user wrote;
that in turn will give us extra confidence that flow analysis produces
the same results in the analyzer and the CFE.

Change-Id: Ie63619a3ca0f011c1ae8e3c0c76b5bc7c1ab8419
Bug: https://github.com/dart-lang/language/issues/4344
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/427341
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>