Simplify error reporting API in Check.CheckContext interface

The CheckContext interface currently defines a method for logging type mismatch that accepts the following parameters:

*) a diagnostic position
*) a found type
*) an expected type
*) a details diagnostic

The report method is used in two ways (mostly orthogonal):

1) simple type mismatch error (of the kind found/req) are reported by using the found/req parameters and by setting the diagnostic to null

2) complex mismatch errors (i.e. caused by inference failures) do not use found/req - they only rely on the diagnostic param.

CheckContext's interface for reporting errors should be simplified to only accept a diagnostic (reason as to why a type mismatch has occurred). Info about found/req types, where available, should be encapsulated inside the details diagnostic.