Macro. Extract _MacroDiagnosticsReporter, _MacroTypeAnnotationLocationConverter.

...and split long methods into smaller ones.

Change-Id: I7bdd5ea7851000ef0fd27735b90eed5199186e13
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/352101
Commit-Queue: Konstantin Shcheglov <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>

Author: scheglov