Replace ANTLR 4 msbuild integration with prebuild step.

Mono's xbuild does not support ItemDefinitionGroup
elements, which the ANTLR 4 msbuild integration uses.

Keep the ANTLR4 msbuild package for the ANTLR jar
but use a custom prebuild step that runs ANTLR.

Also commit files generated by ANTLR to the repo.

The ANTLR build nuget package can be removed from
the repo now and we can rely on package restore.