Recreate old decorator metadata behavior

[weswigham]

Fixes #18509.

With strictNullChecks off, now we intentionally elide undefined and null from the list of type nodes we look at when attempting to find a metadata type for a union type node (and always elide never), emulating our pre-2.4 behavior. As stated in the issue, the behavioral change was actually witnessable in 0eaa8eb - you can see this PR reverts the baselines changed in that commit to their original output.

[sheetalkamat]

@mhegazy had mentioned to not treat null | undefined as excpetion in #13540 (comment) I do think the change makes sense yet i would let @mhegazy approve this.

[typeofweb]

@weswigham What's the reasoning behind not eliding null and undefined from unions for metadata when strictNullChecks is on?

[weswigham]

@mmiszy We're using the syntactic data to simulate how the typesystem sees the type. "Complex" types (unions, intersections, etc) are emitted as Object - unions with null and undefined are no exception. The difference is that outside of strict, we elide null and undefined from unions for the purpose of comparison, so we need to emulate that behavior here to be consistent with how the typesystem views the types.