Instead of caching only the default formatter, cache the last used
formatter if possible.

This is better because it's a common use case to create a formatter
in a different language and reuse it a lot, rather than create
several formatters in various languages.

Running the following benchmark:
```
let i = 0;

function toLocaleString() {
  i++;
  return i.toLocaleString();
}

i = 0;
function toLocaleStringWithLocale() {
  i++;
  return i.toLocaleString('en-US');
}

const functions = [toLocaleString, toLocaleStringWithLocale];

for (const f of functions) {
  let start = performance.now();
  for (let i = 0; i < 10e5; i++) {
    f();
  }
  let end = performance.now();
  print(`${f.name}: ${end - start}`);
}
```
sees the following improvements:

With this patch:
  toLocaleString: 384.292
  toLocaleStringWithLocale: 450.48900000000003

Without this patch:
  toLocaleString: 341.952
  toLocaleStringWithLocale: 23439.694

This a little over 50x improvement.

Bug: chromium:926075
Change-Id: I0e316e959c90243e175df985854832a7abddbf54
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2536461
Commit-Queue: Sathya Gunasekaran  <gsathya@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71188}