Bug#45288: pb2 returns a lot of compilation warnings

Fix assorted warnings that are generated in optimized builds.
Most of it is silencing variables that are set but unused.

This patch also introduces the MY_ASSERT_UNREACHABLE macro
which helps the compiler to deduce that a certain piece of
code is unreachable.

include/my_compiler.h:
  Use GCC's __builtin_unreachable if available. It allows
  GCC to deduce the unreachability of certain code paths,
  thus avoiding warnings that, for example, accused that a
  variable could be used without being initialized (due to
  unreachable code paths).

sql/sql_partition.cc

@@ -6786,8 +6786,8 @@ int get_part_iter_for_interval_via_mapping(partition_info *part_info,
     }
   }
   else
-    assert(0);
-  
+    MY_ASSERT_UNREACHABLE();
+
   can_match_multiple_values= (flags || !min_value || !max_value ||
                               memcmp(min_value, max_value, field_len));
   if (can_match_multiple_values &&