This change decouples gr-diff-builder and gr-syntax-layer. gr-syntax- layer will now live in gr-diff-host. As a result, from the perspective of gr-diff/gr-diff-builder, plugin layers and the syntax layer will now be part of the same list. A future potential refactor could also lift gr-coverage-layer instances in a similar fashion. Change-Id: Ic10b45d70a59c2e18d3b408880f66251ab93e06e