Make simplify replace type parameters inside method types

[odersky]

Simplify usually replaces a constrained TypeParamRef with the associated TypeVar, which is
necessary so that the TypeParamRef is properly instantiated afterwards. But it did not
recurse inside method types, since that might change signatures.

This commit adds an auxiliary TypeMap over method types that just instantiates
TypeParamRefs without applying any of the other transformations of simplify.

Fixes #15428

[smarter]

I don't understand why this would be needed, the TypeVar should be permanently instantiated by the time we try to pickle it, if the instantiation was rolled back, that points to a bug somewhere (maybe a missing TyperState#commit() ?)

[smarter]

Ah I misunderstood, it's not the TypeVar that we replace, it's the TypeParamRef, and we get a TypeParamRef because the constraint is generated from the bounds of the method type. But then we have to be really careful about always calling simplify on every part of the type, for example I'm not sure if this is actually done in AppliedType which are normalized:
https://github.com/lampepfl/dotty/blob/741587d8e1c0238ca9c0986a77519cdf5a6f2fa8/compiler/src/dotty/tools/dotc/core/TypeOps.scala#L147-L148
Should we call simplify(normed, theMap)?

[odersky]

Not sure about it. So far, everything worked with just simplify. It's just that simplify was not applied inside MethodTypes. The PR fixes that concrete problem. I'd wait with the rest until we have test cases.