glsl: lower gl_TessLevel* from float[n] to vecn.

Similar to gl_ClipDistance -> gl_ClipDistanceMESA

v2: - renamed is_mesa_var to lowered_builtin_array_variable
    - moved LowerTessLevel into gl_constants
    - cosmetic changes in lower_tess_level.cpp

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
diff --git a/src/glsl/linker.cpp b/src/glsl/linker.cpp
index 498e8d5..b9e443a 100644
--- a/src/glsl/linker.cpp
+++ b/src/glsl/linker.cpp
@@ -3324,6 +3324,10 @@
          lower_clip_distance(prog->_LinkedShaders[i]);
       }
 
+      if (ctx->Const.LowerTessLevel) {
+         lower_tess_level(prog->_LinkedShaders[i]);
+      }
+
       while (do_common_optimization(prog->_LinkedShaders[i]->ir, true, false,
                                     &ctx->Const.ShaderCompilerOptions[i],
                                     ctx->Const.NativeIntegers))