glsl/linker: fix location for array subscript

Pierre-Eric Pelloux-Prayer requested to merge pepp/mesa:glsl_fix_xfb_arrays into main

tfeedback_decl::assign_location assumes that arrays are going to be packed. But some conditions might prevent packing (e.g: explicit location or smooth interpolation mode).

Instead of assuming that packing will happen, this commit adds a check to determine if it'll happen and use the result to compute the proper location.

Closes: #2214 (closed)

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