Do not warn about expected missing positions in quotes.reflect.Symbol

[jchyb]

Related to #19250

In the PR above it is said that trees and symbols always have to have a position. I think certain cases of the quotes.reflect.Symbol.pos method might have been overlooked, since the doc for Symbol.span (internally in the compiler) explicitly states that the Symbol might not have a position assigned if it was not loaded from source or TASTy. It is true however for quotes.reflect.Tree.pos - the trees always have a position since they have to be loaded from source or TASTy.

Thankfully, quotes.reflect.Symbol.pos returns Option[Symbol], so we can just return None in the cases where the position does not exist. Previously we would only return None if the symbol itself did not exist, however this was not specified anywhere in the Quotes docs, so I think it's fine to change that now (or at least better then any other alternatives), even if this change would go into 3.3.5.

As a comparison, the output of println(TypeRepr.of[String].typeSymbol.pos.toString) would be:
Before #19250: Some(?) (Some(NoSource) without set span/position - can cause crashes)
After #19250: Some(?) (Some(NoSource) with span/position - can still cause crashes)
After this PR: None

Fixes #21672

[jchyb]

@dwijnand Thank you so much for the review! I've rebased for the purpose of retesting, but haven't made any other changes since