abseil-duration-addition

Check for cases where addition should be performed in the absl::Time domain. When adding two values, and one is known to be an absl::Time, we can infer that the other should be interpreted as an absl::Duration of a similar scale, and make that inference explicit.

Examples:

// Original - Addition in the integer domain
int x;
absl::Time t;
int result = absl::ToUnixSeconds(t) + x;

// Suggestion - Addition in the absl::Time domain
int result = absl::ToUnixSeconds(t + absl::Seconds(x));