[analyzer] Move some error reporting code into _fe_analyzer_shared.

The following classes are moved from `package:analyzer` to
`package:_fe_analyzer_shared`:

- `Diagnostic`
- `DiagnosticMessage`
- `DiagnosticMessageImpl`

The following declarations are also moved, since they are needed by
the above classes:

- `formatList`
- `Severity`
- `Source`
- `TimestampedData`

There is no change to the analyzer public API, and `export`
declarations have been added to the analyzer libraries that the
declarations have been moved from, so that code depending on these
declarations is unaffected.

These changes are part of a larger arc of work that introduces methods
`.withArguments` and `.at`, forming a literate API for reporting
analyzer errors that looks roughly like this:

    diagnosticReporter.reportError(
        ERROR_CODE.withArguments(...arguments...).at(...location...));

Moving this code into `_fe_analyzer_shared` is necessary because
scanner error codes are defined inside `_fe_analyzer_shared` (to allow
the scanner to be shared between the analyzer and CFE). Hence, to
avoid a circular depedency between `_fe_analyzer_shared` and
`analyzer`, the `.withArguments` and `.at` methods will need to live
in `_fe_analyzer_shared` too, as well as the classes representing the
diagnostic messages they create.

Note that there are some minor changes to
`pkg/analysis_server_plugin/api.txt` and
`pkg/analyzer_plugin/api.txt`; these have to do with the way the
`api.txt` generator chooses to report referenced elements, and don't
reflect actual API changes.

Change-Id: I6a6a6964a5c46f4a0205ce0d85620669ce55eb3c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/444620
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

https://dart.googlesource.com/sdk/+/8b23e3449d7335d42f5abeb1f973d46abc0c5b8d