SI-7694 Disable type constraint checking for synthetic TypeTrees

Which might be based on slack LUBs that fail to capture constraints
between type parameters.

This slackness is noted in `mergePrefixAndArgs`:

// Martin: I removed this, because incomplete. Not sure there is a
// good way to fix it. For the moment we just err on the conservative
// side, i.e. with a bound that is too high.