Cyclic reference when incrementally compiling Dotty

[smarter]

This is similar to #6460 in that the cycle involves tpd/Trees.Instance/Types, but it likely has a different root cause, it can be reproduced by separately compiling a few files from Dotty with Dotty itself on the classpath:

> dotty-compiler-bootstrapped/run -with-compiler compiler/src/dotty/tools/dotc/ast/Desugar.scala compiler/src/dotty/tools/dotc/ast/Trees.scala compiler/src/dotty/tools/dotc/core/Types.scala

-- [E046] Cyclic Error: compiler/src/dotty/tools/dotc/core/Types.scala:22:7 ----
22 |import ast.tpd._
   |       ^
   |       Cyclic reference involving constructor Instance