Fix warnings in ImmutableSetTest and SequenceTest.

Doing "I++" inside of an EXPECT_* triggers

  warning: expression with side effects has no effect in an unevaluated context

because EXPECT_* partially expands to

  EqHelper<(sizeof(::testing::internal::IsNullLiteralHelper(i++)) == 1)>

which is an unevaluated context.

llvm-svn: 275717