Metal: Fix rewritten out variables with underscores Fix compilation in case of output variables start with underscores. Make name emission always emit MSL name ANGLE_{name}, so that GLSL `_e` and `e` cannot clash. This regressed in angleproject:8558. Bug: chromium:335744344 Change-Id: Ibae4dba4a24888acc1461582e69d48218ba11176 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/5473412 Reviewed-by: Kenneth Russell <kbr@chromium.org> Auto-Submit: Kimmo Kinnunen <kkinnunen@apple.com> Commit-Queue: Kimmo Kinnunen <kkinnunen@apple.com> Reviewed-by: Kimmo Kinnunen <kkinnunen@apple.com>