This patch to the Go frontend by Than McIntosh adds the "no nil check
needed" annotation to the dereference operations created in
Parse::enclosing_var_reference (this is safe since the closure object
is under control of the compiler, and pointer fields in it will always
be non-nil).  Bootstrapped and ran Go testsuite on
x86_64-pc-linux-gnu.  Committed to mainline.

Ian