types(reactivity): fix corner case on UnwrapRef

[yoyo930021]

Same as vuejs/core#614

We need special string or number types sometimes.
Ex. RelativePath, AbsolutePath

type AbsolutePath = string & { __brand: unknown }
function readFile (path: AbsolutePath) {
  // do some thing
}
function getAbsolutePath (path: string) {
  if (path.startWith('file://')) return path as AbsolutePath
}

I use it for safe use translation keys,
but code is broken when passing UnwrapRef on vetur template interpolation.

[LinusBorg]

@ktsn What do you think about this PR?

[ktsn]

LGTM.