Fix #13440: Avoid capturing var in message closure

[griggt]

Fixes #13440

[som-snytt]

With this branch, I get

java.lang.NullPointerException: Cannot invoke "dotty.tools.dotc.core.Names$SimpleName.length()" because the return value of "dotty.tools.dotc.parsing.Scanners$Scanner.name()" is null while compiling keyword.scala

for

val s = s"$enum"

[nicolasstucki]

@griggt could you rebase this PR? I want to test a theory, it might sole the source-link-test.

[som-snytt]

I see you ignored my comment, but did you actually try it?

[griggt]

but did you actually try it?

Was this question for me? Yes, I tried it. It looks like you uncovered additional issues with the existing implementation.

This was just intended as a quickie PR to fix the original issue as reported in #13440. It seems that #13442 tackles those additional issues.