compiler: adjust closure field indexes
compiler: adjust closure field indexes

This corrects the code that handles composite literals where
the key is a variable in an enclosing function.  See bug475.go
in the testsuite.

R=iant
CC=gofrontend-dev
https://golang.org/cl/11390043

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