Improve diagnostics on header/impl classes when scopes don't match

Try to report most of the errors on the actual members of the impl
class. In many cases, there's a 1:1 mapping of header to impl class
members, so the error "some members are not implemented" on the class
declaration itself is redundant. Exceptions include functions/properties
from supertypes (there may be no other place to report a signature
mismatch error in this case), functions/properties not marked with
'impl' (the checker is only run for declarations explicitly marked with
'impl') and default constructors (the checker is not run for them)

 #KT-18447 Fixed

compiler/testData/multiplatform/missingOverload/output.txt

@@ -11,7 +11,7 @@ The following declaration is incompatible because parameter types are different:
 
 header fun g(s: String)
 ^
-compiler/testData/multiplatform/missingOverload/jvm.kt:1:12: error: 'impl' class 'Foo' has no implementations of 'header' class members:
+compiler/testData/multiplatform/missingOverload/jvm.kt:1:12: error: 'impl' class 'Foo' has no implementation of 'header' class members:
 
     public final header fun f(a: Any): Unit