Translator: Mark redeclared gl_Position/PointSize as built-in When redeclared according to EXT_separate_shader_objects, these variables were given a UserDefined symbol type. While this doesn't affect SPIR-V (with Vulkan being the only backend that supports EXT_separate_shader_objects), it does affect the rest of the AST, including GLSL generation which would output _ugl_Position if support for the extension is enabled. Bug: angleproject:42265086 Bug: angleproject:349994211 Change-Id: I04d088017dbe6e32bfee675f0124c4cc3ffdefc2 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6985322 Auto-Submit: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>