Internal cross reference to a section title starting with a backtick

[stenzengel]

If a section title starts with a backtick, a cross reference to it is not resolved and no warning is emitted.

Example:

= Test
:attribute-missing: warn

== `String`

== Class `String`

== Referencing Section

<<`String`>>
<<Class `String`>>
<<Unexisting>>

Current behavior:

The generated HTML for the "Referencing Section" looks like this:

<p>&lt;&lt;`String`&gt;&gt;
<a href="#_class_string">Class <code>String</code></a>
<a href="#Unexisting">[Unexisting]</a></p>

I.e. the cross reference to the section starting with a backtick is not interpreted and no link tag is created.

Asciidoctor only emits the info message for the reference to "Unexisting":

asciidoctor: INFO: possible invalid reference: Unexisting

Expected behavior:

Preferred solution: The reference is interpreted and an a tag is created
Alternative: An info message is created (as for "Unexisting")

I'm not sure if this is an error, or if this is intended behavior. Perhaps there is another way to reference a section title starting with a backtick? I know about referencing by ID, but in this case I prefer referencing by title.

This issue seems to be different than #3642, because backticks in titles seem to work in general: Referencing the other section containing backticks (but not starting with backticks) works correctly.