Support type narrowing by v-if on template diagnostic

[ktsn]

Like TypeScript's if statement, v-if, v-else-if and v-else should narrow union types which are used in their conditions.

<template>
  <p v-if="!messages">Loading...</p>
  <div v-else> <!-- messages should be narrowed to string[] here -->
    <p v-for="m in messages" :key="m">
      {{ m }}
    </p>
  </div>
</template>

<script lang="ts">
import Vue from 'vue'

export default Vue.extend({
  data() {
    messages: null as string[] | null
  }
})
</script>