Avoid NoPosition.start in Scaladoc warning

[som-snytt]

Some docs for synthetic symbols derived from library-aux sources have no position (for some reason).

Also for some reason, operations on position are brittle instead of safe. I'd expect to map a position such that the degenerate position remains safely degenerate instead of throwing (which helps no one). After solving NoSymbol has no owner, they ought to have tackled NoPosition has no start.

This commit just patches the warning not to do position arithmetic on NoPosition.

It requires restarr to work for the OP.

Edit: this commit also doesn't address "doc variable resolution in code block".

Edit: Fixes scala/bug#12964

[SethTisue]

It requires restarr to work for the OP.

And that's why it makes sense to squeeze this into 2.13.14?

[som-snytt]

This is a blocker for work on the PR for documentation of Any#asInstanceOf.

At least, it's a mental blocker.

[lrytz]

degenerate position remains safely degenerate instead of throwing

Not sure for start / point / end that return Int. What should they do? pos.withPoint(pos.start + vstart + 1) giving an arbitrary position is certainly harder to debug than crashing.

This PR LGTM.

[som-snytt]

Tested manually with restarr, by adding a doc var in the code block in the doc for Any.

--- a/src/library-aux/scala/Any.scala
+++ b/src/library-aux/scala/Any.scala
@@ -29,6 +29,7 @@ package scala
  *
  *     val w = new Wrapper(3)
  *     w.print()
+ *     val x = s"Sample bad doc macro in code block: $x"
  * }}}
  *
  * See the [[https://docs.scala-lang.org/overviews/core/value-classes.html Value Classes and Universal Traits]] for more

then library/doc shows a weirdly unpositioned message (but doesn't crash):

[warn] .../scala/src/library/scala/collection/LinearSeq.scala:48:7: The comment for method head contains @inheritdoc, but no parent comment is available to inherit from.
[warn]   def head: A
[warn]       ^
[warn] Variable x undefined in comment for class Any in class Any
[warn] .../scala/src/library/scala/math/Ordering.scala:548:10: Variable doubleOrdering undefined in comment for trait IeeeOrdering in trait IeeeOrdering
[warn]       * $doubleOrdering
[warn]          ^

I confirmed that the diff crashes as reported with untweaked 2.13.x. What starr is that.

Just remembered "What star is that?" was my first astronomy guide.
https://www.abebooks.com/book-search/title/what-star-is-that/author/peter-lancaster-brown/