Reduce the use of `<unnamed>` in analyzer error messages.

Two error messages are affected:

1. When the user tries to access a static member of an extension
through an instance of a class that mentions the extension's "on"
type, and that extension is unnamed, we now report
INSTANCE_ACCESS_TO_STATIC_MEMBER_OF_UNNAMED_EXTENSION instead of
INSTANCE_ACCESS_TO_STATIC_MEMBER.  The new error message is the same
as the old one (and uses the same sharedName), but it omits the
correction message suggesting to the user that they try accessing the
static method directly via the extension name, since this advice
doesn't apply.

2. When we report AMBIGUOUS_EXTENSION_MEMBER_ACCESS, if one of the
ambiguous members comes from an unnamed extension, we now report the
unnamed extension as "unnamed extension on '$type'" rather than
referring it to as an extension named `<unnamed>`.

Change-Id: I3ca3a1ccc9399b26b083040de20db8e4f691be32
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/217102
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>