This patch to the Go frontend fixes the case where a struct composite
literal uses a field name which happens to be a predeclared name.  The
problem was that predeclared names aren't packed as hidden names,
because the package name is not known when they are declared.  The field
names in structs are, however, packed when appropriate.  Bootstrapped
and ran Go testsuite on x86_64-unknown-linux-gnu.  Committed to
mainline.

Ian