Checking the receiver name

[Jonathan Gibbons]

Werner,

As part of the JCReceiverVariableDecl cleanup, I deliberately removed a 
check on the validity of the name of the receiver parameter.

This was for number of reasons
-- the tests (i.e. in test/**) were failing after I added the call to 
Attr the name in MemberEnter
-- the check in Attr looked dubious, using string equality
-- the check in Attr is happening at the wrong time -- it needs to 
happen before annotation processing

That being said, I think we do need a check, but maybe much sooner after 
we Attr the name.  This morning it occurred to me that the check should 
be to conpare the declared type of the receiver parameter with the type 
of the name expression.

Does that make sense? If so, I'll go ahead and add the check, and 
restore the appropriate diags examples, etc.

-- Jon