Run of tildes in HTML comments confuses/reverses the at-directive parser

[amartini51]

The at-directive parser looks for runs of ~ or ` to delimit the start and end of code blocks. It knows that an at-directive can't start by @foo inside a code block. However, if one of those lines appears in an HTML comment, the at-directive parser doesn't ignore the line. This results in the "not in a code block" logic being inverted. For example, we ran into this issue in "The Swift Programming Language" (reduced example):

<!--
  Adding Child Tasks to a Task Group
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  
  - Creating a group with ``withTaskGroup`` and ``withThrowingTaskGroup``
  
  - awaiting ``withGroup`` means waiting for all child tasks to complete
-->

```
struct NonsendableTemperatureReading {
    var measurement: Int
}

@available(*, unavailable)
extension NonsendableTemperatureReading: Sendable { }
```

In this case, @available is treated as the start of an at-directive and the code listing is mis-parsed and displays as follows:

rdar://101828693

[amartini51]

Thanks to @QuietMisdreavus for investigating this issue with me!