buildifier: make formatting idempotent for compact block statements followed by comments (#1481)

* Make formatting idempotent for compact block statements followed by comments

Comments following a block statement (def/for/if) now parse as standalone CommentBlock statements, matching the expanded-form parse, so buildifier converges in a single pass. Fixes bazelbuild/buildtools#1470.

* Regenerate parse.y.go with goyacc (x/tools v0.15.0)

* Add regression testdata for #1470 (input)

* Add regression testdata for #1470 (golden)

Add golden file 078.golden with function definitions and comments.