SI-7694 Mark temporaries in NamesDefaults as SYNTHETIC

This will enable the subsequent commit to loosen type bound
checking for these as a way to avoid slack inferred LUBs
preventing compilation.
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.

"synthetic" is taken to be a TypeTree without an original tree where
the current owner is synthetic. Omitting the second condition
results in a test failure for neg/t0513.scala.