Types.isSubtype takes wildcards as input

Due to an incorrect substitution (I think), bounds checking sometimes invokes Types.isSubtype with a wildcard in place of a type. We work around this by getting the wildcard's bound, but that should not be subtyping's job, and is probably incorrect behavior.

I encountered this while building JDK & running tests after removing the wildcard lower bound mapping performed in the initialization of 'lower'. (I kept the capture variable lower bound mapping, which is correct.)