Use backend interface for interface method tables
Merge remote-tracking branch 'upstream/master' into gccgo_interface_method_tables
Addressed 7 comments from codereview. ::do_types might be wrong.
Merge branch 'gccgo_inteface_method_tables' of github.com:paranoiacblack/gcc into gccgo_interface_method_tables
Addressed comments on get_tree for method tables.

* Removed constructor_expression from this change.
https://codereview.appspot.com/64830043/#msg5/
https://codereview.appspot.com/64830043/#msg7
compiler: Use backend interface for interface method table expressions.
Merge branch 'gccgo_interface_method_tables' of github.com:paranoiacblack/gcc into gccgo_interface_method_tables
compiler: Use backend interface for interface method table expressions.