Cleanup: enable and silence Clang-Tidy readability-function-size

Enable Clang-Tidy's `readability-function-size` rule and add a few
`NOLINT` markers to explicitly silence warnings for three functions.
These functions are huge and would IMO benefit from splitting up, but
are hard to without intimate knowledge of the code.

At least by enabling the rule, we can start tweaking the values and
refactoring other functions that bubble up as being too long/complex.

No functional changes.

.clang-tidy

@@ -14,8 +14,6 @@ Checks:  >
 
   -readability-inconsistent-declaration-parameter-name,
   -readability-redundant-preprocessor,
-  -readability-function-size,
-  -readability-function-size,
   -readability-redundant-member-init,
   -readability-const-return-type,
   -readability-static-accessed-through-instance,

source/blender/blenkernel/intern/mesh_validate.c

@@ -214,6 +214,7 @@ static int search_polyloop_cmp(const void *v1, const void *v2)
  *
  * \return false if no changes needed to be made.
  */
+/* NOLINTNEXTLINE: readability-function-size */
 bool BKE_mesh_validate_arrays(Mesh *mesh,
                               MVert *mverts,
                               unsigned int totvert,

source/blender/modifiers/intern/MOD_solidify_extrude.c

@@ -182,7 +182,7 @@ static void mesh_calc_hq_normal(Mesh *mesh, float (*poly_nors)[3], float (*r_ver
 /* -------------------------------------------------------------------- */
 /** \name Main Solidify Function
  * \{ */
-
+/* NOLINTNEXTLINE: readability-function-size */
 Mesh *MOD_solidify_extrude_modifyMesh(ModifierData *md, const ModifierEvalContext *ctx, Mesh *mesh)
 {
   Mesh *result;

source/blender/modifiers/intern/MOD_solidify_nonmanifold.c

@@ -132,6 +132,7 @@ static int comp_float_int_pair(const void *a, const void *b)
   return (int)(x->angle > y->angle) - (int)(x->angle < y->angle);
 }
 
+/* NOLINTNEXTLINE: readability-function-size */
 Mesh *MOD_solidify_nonmanifold_modifyMesh(ModifierData *md,
                                           const ModifierEvalContext *ctx,
                                           Mesh *mesh)