Tagged most functions in absl/time/time.h as ABSL_ATTRIBUTE_CONST_FUNCTION or ABSL_ATTRIBUTE_PURE_FUNCTION

However, both absl_attributes are now unimplemented to avoid breaking existing users.

PiperOrigin-RevId: 496769399
Change-Id: I9c00cb60b885526300d744f9ea7c0f2178f092bb