7175911: Simplify error reporting API in Check.CheckContext interface
Summary: Make error messages generated during Check.checkType more uniform and more scalable
Reviewed-by: jjg, dlsmith

7151492: Encapsulate check logic into Attr.ResultInfo
Summary: ResultInfo class should be used to make attribution code transparent w.r.t. check logic being used
Reviewed-by: jjg, dlsmith

6886247: regression: javac crashes with an assertion error in Attr.java
Summary: capture conversion does not work on nested types
Reviewed-by: jjg