[ts] Support private methods overloads

[nicolo-ribaudo]

Q	A
Fixed Issues?	Fixes #13875
Patch: Bug Fix?	Yes
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/49364/

[JLHwung]

TS currently throws when class method is missing implementation. We can use declare class here.

And TS also throws for non-private overload.

class C {
  z(): string;
  z(): number;
}

[nicolo-ribaudo]

It reports the same error for function declarations:

function a(): number;

I think we can continue considering this a type-checker level error as we have been doing so far, since it doesn't affect the compiler output in any way.

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