Add features to dump and replace shader source. dump_shader_source writes shader source strings to files named with the hash of the source and shader type. enable_shader_substitution allows subsituting the shader source with the strings contained in a file. The same file naming scheme as dump_shader_source is used so shaders can be dumped in one run, modified and subsititued in the next run. Use the ANGLE_SHADER_DUMP_PATH and debug.angle.shader_dump_path environment variables to control where files are written. If they are unspecified, write to the temp directory. Based upon Cody's CLs: https://chromium-review.googlesource.com/c/angle/angle/+/2755841 https://chromium-review.googlesource.com/c/angle/angle/+/3961670 Bug: angleproject:7760, chromium:1385510 Change-Id: I43b9827b977079b88daa794e867637f7a126b080 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4192347 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Cody Northrop <cnorthrop@google.com> Commit-Queue: Geoff Lang <geofflang@chromium.org>