Flags defined in FromStartFlags/AfterLoadFlags should not be (re)set in completers or InfoTransformers

[smarter]

This is a follow-up on #4202:

Here are the remaining flag mutations I believe are problematic but didn't know how to fix:

• https://github.com/lampepfl/dotty/blob/16ce7e46672857a07d46422cd6a1c33aa8dff5f9/compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala#L115
• https://github.com/lampepfl/dotty/blob/16ce7e46672857a07d46422cd6a1c33aa8dff5f9/compiler/src/dotty/tools/dotc/typer/Namer.scala#L754
• https://github.com/lampepfl/dotty/blob/16ce7e46672857a07d46422cd6a1c33aa8dff5f9/compiler/src/dotty/tools/dotc/typer/Namer.scala#L987

Removing any of these flags from FromStartFlags/AfterLoadFlags results in cyclic reference errors, so we can't avoid the problem that way.