Add ABSL_IS_TRIVIALLY_RELOCATABLE and ABSL_ATTRIBUTE_TRIVIAL_ABI macros for use with clang's __is_trivially_relocatable and [[clang::trivial_abi]].

PiperOrigin-RevId: 463668740
Change-Id: I2d2d2f53d8184a7e4f7c848c2a5f5140c2481d72