dump_shader_source: Also dump program links. When the dump_shader_source is enabled, dump a small file for each program link containing which shaders were attached to the program. This allows us to build perf tests knowing which shaders can link together. Bug: angleproject:7760, chromium:1385510 Change-Id: I0713e9f04cc157c3a3eeeec6d597db27b08575d1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4214011 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>