compiler: Handle enclosing vars for function type in function lit.
compiler: Handle enclosing vars for function type in function lit.

This fixes a dumb bug in which the enclosing vars were
incorrectly cleared when a function literal contains a
reference to a function type.  The test for this will go into
the master repository in the change at
https://golang.org/cl/121200043 .

This fixes http://gcc.gnu.org/PR61308 .

R=iant
CC=gofrontend-dev
https://golang.org/cl/122020043

qskycolor/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,golang/gofrontend,golang/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,golang/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,qskycolor/gofrontend,anlhord/gofrontend,anlhord/gofrontend,qskycolor/gofrontend,golang/gofrontend,anlhord/gofrontend