distinct_diff: avoid expression with side effects within typeid

distinct_diff: avoid expression with side effects within typeid

When compiling with clang, the following warning is emitted:

abg-comparison.cc:2674:17: warning: expression with side effects will be
evaluated despite being used as an operand to 'typeid' [-Wpotentially-evaluated-expression]
  return typeid(*first.get()) != typeid(*second.get());
                ^

Mitigate that warning by moving potential side effects out of typeid.

 * src/abg-comparison.cc: fix clang warning "potentially-evaluated-expression"

Signed-off-by: Matthias Maennich <maennich@google.com>