go/types: improve errors for method calls with pointer receivers
The compiler has better error messages for methods called without a
pointer receiver when one is expected. This change is similar to
CL 229801, but for method calls.
Also, added better error messages for functions called with the wrong
capitalization. I left the third TODO in this switch statement almost
as-is because I'm not sure that the extra complexity is worth it -
I adjusted the error to look like the one the compiler reports.