Fix decorator emit for accessors

[rbuckton]

This fixes an issue where the new emitter was not emitting decorators for accessor pairs where only the second accessor was decorated:

declare var dec: any;
class A {
    @dec() set value(value: string) { } // emits fine
    get value() { return ""; }
}
class B {
    set value(value: string) { }
    @dec() get value() { return ""; } // not emitted
}

Fixes #12727

//cc: @mhegazy