fix(compiler-core): comments in the <pre> tag should be removed in production mode

[HcySunYang]

Fix: #2217

[HcySunYang]

The compiler in production mode:

[posva]

Can we add a test that toggles production mode and checks this behavior?

[HcySunYang]

@posva yes I do it later

[FrankFang]

Just ask, why not keep html comments in <pre> ?

[HcySunYang]

By default, comments are not kept in production, but you can achieve this by providing the comments: true option to the compiler

[chenhaihong]

Just ask, why not keep html comments in <pre> ?

The both ones below work with the same as each other in a pure html doc.
So i think that it may remove the html comments in a production mode.

# code a
<pre>
      <!-- <code class="javascript" v-hljs>const a = 100;</code> -->
      <code class="javascript">const a = 100;</code>
</pre>

# code b
<pre>
      <code class="javascript">const a = 100;</code>
</pre>

But the point is to fix the error of compiler.

[yyx990803]

See f411924 - I refactored the logic to avoid duplicated code/iterations.

[JasperTey]

By default, comments are not kept in production, but you can achieve this by providing the comments: true option to the compiler

Where exactly do I specify this comments: true option? Somewhere within my package.json? Within my webpack config "vue-loader" options? Or somewhere else?