Translator: Fix gl_in redeclaration This was seemingly untested, and had multiple issues: * GLSL generation would output `gl_in gl_PerVertex { ... } gl_in;` * SPIR-V generation would redeclare `gl_in` despite one already existing. * If sized after declaration, the redeclared `gl_in` was referencing a different variable than the resized one. Bug: angleproject:42264111 Bug: angleproject:349994211 Bug: angleproject:447556589 Change-Id: Id63912b68625bafcc64197807b1c8a86e6f952b5 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6985323 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yuxin Hu <yuxinhu@google.com>