Fix emission of -Wzero-as-null-pointer-constant when comparing integers.

The following code fails to compile:

#pragma clang diagnostic error "-Wzero-as-null-pointer-constant"
void foo() {
  EXPECT_EQ(0, 0);
}

This happens because gtest checks the first argument to EXPECT_EQ and
ASSERT_EQ is a null pointer constant. The magic it does to do this causes the
warning to be emitted.

This patch removes that check. It replaces the explicit check with a Compare
overload that can only be selected when 0 or nullptr is passed on the LHS
with a pointer on the right.

This patch does not suppress -Wzero-as-null-pointer-constant when users
are actually using it as NULL.

PiperOrigin-RevId: 236654634