[analyzer] Simplify and clean up duplicate declaration checking.

This change reworks
`MemberDuplicateDefinitionVerifier._checkConflictingConstructorAndStatic`
and `MemberDuplicateDefinitionVerifier._checkDuplicateIdentifier` into
a form that is easier to reason about and has slightly faster
performance (measured by instruction count).

In the previous design, two maps were maintained for each scope in
which getters and setters might appear:

- one called `getterScope`, which (confusingly) held getters, setters,
  and method declarations,

- and one called `setterScope` which only held setters.

This was difficult to reason about. In particular, it had a
longstanding bug that was only recently fixed (see
https://dart-review.googlesource.com/c/sdk/+/434061): if a setter was
encountered first, it was stored in `getterScope`, but if a getter was
encountered next, it was necessary to move the setter to `setterScope`
in order to store the getter in `getterScope`.

It was also inefficient, since in many cases, two map lookups were
needed in order to check for both getter and setter conflicts.

In the new design, there is a single map, whose values point to either
a `_ScopeEntryFragment` (in the case where just one declaration of the
given name has been seen) or a `_ScopeEntryGetterSetterPair` (in the
case where both a getter and a setter have been seen).

The new design has modestly better performance when measured by
front_end/tool/benchmarker.dart:

     Comparing snapshot #1 (before.aot) with snapshot #2 (after.aot)
     instructions:u: -0.0192% +/- 0.0131% (-6291379.67 +/- 4292101.91) (32692292436.33 -> 32686001056.67)

Change-Id: Ia6675fa4d3579779e6066db3760d2cc38a2c859e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/435602
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>