Introduce `ExtensionResolutionResult` for use by `ExtensionMemberResolver`.

Previously, `ExtensionMemberResolver` used `ResolutionResult` to
return its results. This created unnecessary complexity, because
`ResolutionResult` needs to be able to represent a lot of resolution
results that aren't possible when resolving extensions (references to
`Function.call`, references to record fields, and various complex
error reporting scenarios). Conversely, `ExtensionResolutionResult`
needs to be able to represent some scenarios that arent relevant when
doing non-extension resolution (extension lookup resulting in no
result or an ambiguous result).

Splitting into two classes makes both classes simpler.

In a few cases, code needs to be able to handle either a
`ResolutionResult` or an `ExtensionResolutionResult`; for these cases,
I've added a common base class `SimpleResolutionResult`.

This change paves the way for some clean-up work I'm doing on
`TypePropertyResolver` (which uses `ResolutionResult`).

Change-Id: I205251e43d7263cd2df7c729d1550dbb55980f62
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393442
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>