When string counters were added, it accidentally broke
transform feedback varyings.

All the strings were combined, resulting in:

  const char *glTransformFeedbackVaryings_varyings_0[] = {
      R"(out_Posout_Aout_Bout_C)"
   };

Instead, generate the counter for the entire group.

This CL results in:

  const char *glTransformFeedbackVaryings_varyings_0[] = {
      R"(out_Pos)",
      R"(out_A)",
      R"(out_B)",
      R"(out_C)",
   };

Test: Manhattan MEC works again
Bug: angleproject:4941
Change-Id: Ie605395942c9105ba234009989f41a2a1cd8c53e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2381565Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Cody Northrop <cnorthrop@google.com>