SI-7694 @uncheckedBounds, an opt-out from type bounds checking

Synthetic defs introduced by transforms like named/default arguments,
ANF (in scala-async) often introduce a type tree (the tpt of the temporary)
that are based on the types of expressions. These types are scrutinized in
RefChecks to check that type parameter bounds are satisfied.

However, the type of the expression might be based on slack a LUB that
fails 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.

The synthesizor can now opt out of bounds by annotating the type as follows:

   val temp: (<expr.tpe> @uncheckedBounds) = expr

WIP