This patch to the Go frontend just moves Selector_expression up in
file.  This is a mechanical change to expressions.cc.  This will make
Selector_expression visible to Builtin_call_expression for later work.
This produces a very large "git --diff", but "git diff --minimal" is
clear.  Bootstrapped and ran Go testsuite on x86_64-pc-linux-gnu.
Committed to mainline.

Ian