Optimize `absl::Duration` division and modulo: Allow the compiler to inline `time_internal::IDivDuration`, by splitting the slow path to a separate function.

With that change, the compiler can inline the fast path. This is specially important in the context of `Duration::operator%=`, because it allows proving that the return value is unused, therefore avoiding expensive multiplies and divides (e.g. `*q = num_hi / den_hi;`).

```
name                         old cpu/op   new cpu/op   delta
BM_Duration_Modulo           23.1ns ± 0%  22.5ns ± 0%   -2.42%  (p=0.000 n=20+16)
BM_Duration_Modulo_FastPath  7.05ns ± 0%  4.85ns ± 0%  -31.17%  (p=0.000 n=20+20)

name                         old time/op          new time/op          delta
BM_Duration_Modulo           23.1ns ± 0%          22.6ns ± 0%   -2.43%  (p=0.000 n=20+16)
BM_Duration_Modulo_FastPath  7.06ns ± 0%          4.86ns ± 0%  -31.18%  (p=0.000 n=20+20)

name                         old INSTRUCTIONS/op  new INSTRUCTIONS/op  delta
BM_Duration_Modulo              188 ± 0%             178 ± 0%   -5.32%  (p=0.000 n=20+20)
BM_Duration_Modulo_FastPath    84.0 ± 0%            62.0 ± 0%  -26.19%  (p=0.000 n=20+20)

name                         old CYCLES/op        new CYCLES/op        delta
BM_Duration_Modulo             73.8 ± 0%            72.1 ± 0%   -2.27%  (p=0.000 n=19+20)
BM_Duration_Modulo_FastPath    22.5 ± 0%            15.5 ± 0%  -31.13%  (p=0.000 n=19+20)
```

Note: We don't need to expose `absl::time_internal::IDivDuration` at all given that we have a public `absl::IDivDuration`.
PiperOrigin-RevId: 610710635
Change-Id: Ief7c3d5b1c000b397d931e9249edcaef96e7151e