add support for eliding the composite literal type name

[calebdoxsey]

You are allowed to elide the composite literal type name for a map:

var m = map[x]x{{}: {}}

Example Program: https://play.golang.org/p/TYRSlXKKxU

Currently godef gets confused by this. I believe this is because it was a relatively recent addition to the language made after godef was initially created: golang/go@492ac4b

I believe these changes fix the problem.