Allow sysimage build without the doc system (#53533)
The earliest bootstrapping code has a definition of `atdoc` that is just
supposed to ignore the doc string and pass the defining code through.
This function is then replaced by the actual docsystem once that is
available. For testing, I wanted to build the whole system image without
the doc system using this boostrap definition. However, this turns out
not to be possible, because there's a few doc syntax semantics that do
not actually just ignore the doc string.
In particular:
```
"""
I am a doc for a particular signature
"""
foo(x::Int, y::Float64)
```
Does not acutally result in a call to `foo`.
And similarly
```
"""
I am a doc for a global binding
"""
MyModule.foo
```
Does not require `MyModule.foo` to actually have a value, since it only
documents the binding.
This PR allows both of those cases in the boostrap version of `atdoc` so
that we can bootstrap without the doc system if we wanted to.
