Filter out synthetic variables from with scopes

Filter out synthetic variables from with scopes

This patch ensures that variables like .new_target aren't overwritable
using with scopes. It does this by ensuring that scope analysis does
not consider with scopes (or eval scopes) for such 'synthetic variables',
similarly to how the 'this' variable was already handled.
The patch also adds a DCHECK for the dynamic parallel to this case,
replacing a previous unreachable path for a particular instance.

BUG=v8:5405