Follow-up to https://github.com/apple/swift/pull/65048
`getDesugaredType` unwraps sugar types that appear in sequence,
to remove sugar from nested positions we need to get a canonical type.
Thanks to @slavapestov for pointing it out.
Generic type aliases, unless desugared, could bring unrelated type variables
into the scope i.e. `TypeAlias<$T, $U>.Context` is actually `_Context<$U>`.
These variables could be inferrable only after the the body the closure is
solved. To avoid that, let's adjust `TypeVariableRefFinder` to desugar types
before collecting referenced type variables.
Resolves: rdar://107835060
