go/parser: fix (pathological) corner case

Inside a control clause (if ... {}), composite
literals starting with a type name must be parenthesized.
A composite literal used in the array length expression
of an array composite literal is already parenthesized.
Not a valid program, but syntactically is should
be accepted.

LGTM=adonovan
R=adonovan
CC=golang-codereviews
https://codereview.appspot.com/142760043