The GL_EXT_separate_shader_objects section of gl.xml has duplicate
entries for glProgramUniformMatrix4fvEXT. Remove that locally, while
this khronos bug -
 https://github.com/KhronosGroup/OpenGL-Registry/issues/443
tracks the upstream fix.

Also add an assert in the script to error out when duplicates exist.

Bug: angleproject:3570
Change-Id: Id0799b0e4f5ec6bc6beb3e18b5f888afec8ac169
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2539119
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>