Change the macro ABSL_IS_TRIVIALLY_RELOCATABLE into a type trait - absl::is_trivially_relocatable - and move it from optimization.h to type_traits.h.

Example of how to change to the new type trait:
```
#include "absl/base/attributes.h"

if (ABSL_IS_TRIVIALLY_RELOCATABLE(T)) { DoSomething(); }
```
to
```
#include "absl/meta/type_traits.h"

if (absl::is_trivially_relocatable<T>::value) { DoSomething(); }
```

Note that optimization.h is also built in C mode so we can't put a type trait there.

PiperOrigin-RevId: 475633715
Change-Id: I2bc85f3f6711d1280049fd01eb97c497d2d0d929