[no-base-to-string] still many false positive with union values

[HolgerJeromin]

Thanks for #1969 :-)
Found a real bug while testing no-base-to-string :-)

Repro

{
  "rules": {
    "@typescript-eslint/no-base-to-string": ["error"]
  }
}

((myvar1: string | number | boolean) => {
    myvar1.toString();
})('');
((myvar1: string | number | boolean | null) => {
    myvar1.toString();
})('');
((myvar1: number | boolean | null) => {
    myvar1.toString();
})(true);

Expected Result
should lint without problems
Actual Result

warning @typescript-eslint/no-base-to-string : 'myvar1 may evaluate to '[object Object]' when stringified.

for all of them

Versions

package	version
@typescript-eslint/eslint-plugin	2.31.0
@typescript-eslint/parser	2.31.0
TypeScript	3.8.3
ESLint	6.8.0
node	X.Y.Z
npm	X.Y.Z

[bradzacher]

any union with null will fail, because null has no toString definition.
It was also specifically not handled on purpose, because null stringifies as "null", which is not a good thing to leak into strings - you should be handling the null state.

If you are okay with null leaking into your strings, you can ignore it with the ignoredTypeNames option.

---

Unions are handled in this rule implicitly - the rule gets all definitions of the toString method for the type. This "just works" for union types, and returns one declaration for each type.

Unfortunately, the previously workaround that's been implemented for the boolean type doesn't implicitly work for unions...

[auhsoja]

Not sure if I handled this in the most technically-correct way. However, I've submitted a PR that should make the errors go away if merged.