Check all usages of lazy val in fields in the codebase

[smarter]

#2506 fixes race conditions in dotty recently introduced when we copied files from scala-reflect, but it does not attempt to look at all usages of lazy val in these files or other files imported before that. We should do a review of all our lazy val fields and decide for each one of them:

• Keep them as-is, but clearly mark the class as not thread-safe
• Make the class thread-safe by adding @volatile
• Use a val or a def instead.

[odersky]

lazy vals are now thread-safe, so this is no longer an issue.