go.tools/ssa: fix a bug building SSA code for ast.CompositeLit.

Map literals should use the same recursion logic as
struct/array/slice literals to apply an implicit &-operator to
the nested literals when a pointer is wanted.

+ test.

Also:
- ensure we set the source location for all Lookup and
  MapUpdate instructions.
- remove obsolete address.object field.

R=gri, crawshaw
CC=golang-dev
https://golang.org/cl/12787048
4 files changed