Allow param decorators to use function generics

[ivogabe]

At the moment, parameter decorators cannot use generics of their parent function. Example:

declare function decorate<T>(): any;
class Foo {
    bar<U>(@decorate<U>() param: U) { }
}

Throws: TS2304: Cannot find name 'U'.
Suggestion: allow using function generics in parameter decorators. The example should compile.

[mhegazy]

But what would that mean.. the decorator does not have access to the installation. the decorator runs only once at the class construction time, so at this point U is just any.

[ivogabe]

Well, that's a good question actually.. I was thinking about #2686, but that seems to be the only use-case of it. Closing this, since #2686 contains that already.