[cfe] Move Builder creation to helper methods

This simplifies the massive switch over fragments by moving each case implementation to its own helper method.

Change-Id: I38df8ed6106c5160eb3a033cd17f831be99de339
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/418101
Commit-Queue: Johnni Winther <[email protected]>
Reviewed-by: Jens Johansen <[email protected]>

Author: johnniwinther