Shared analysis: API changes for pattern variables.

This change is part of a process of shifting the responsibility of
tracking the set of variables defined by a pattern into the shared
analysis logic.  A `variableName` parameter is added to
`analyzeDeclaredVariablePattern`, and a field `componentVariables` is
added to `MatchContext` to track the set of variables associated with
each variable name in a pattern.  In the CLs that follow, I plan to
use this information in the place of the `getJoinedVariableComponents`
method.  This will give the client more flexibility in deciding when
and how to join variables in logical-or patterns.  (This is needed
because the CFE joins variables eagerly as a necessary part of
lowering, whereas the analyzer joins variables only at the end of the
pattern, to improve the quality of error messages).

Also, a `variables` parameter is added to `analyzeIfCaseElement` (to
make it consistent with `analyzeIfCaseStatement`), and the
`patternVariables` parameter is removed from `analyzePatternForIn`.
With these changes, we now consistently have a `variables` map in
situations where the pattern match is refutable (and thus logical-or
patterns are allowed), and we have no `variables` map in situations
where the pattern match is irrefutable (and thus no variable joining
is needed).

Change-Id: I1c511d12e827c52ef9bd8b7ff2b561ea9713a932
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279239
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

https://dart.googlesource.com/sdk/+/c498870e85715c1d7da276d1f08157168885c010