compiler: move Selector_expression up in file

This is a mechanical change to move Selector_expression up in expressions.cc.
This will make it visible to Builtin_call_expression for later work.
This produces a very large "git --diff", but "git diff --minimal" is clear.

Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/536642

gcc/go/gofrontend/MERGE

-3c2a441ef6cafb018bb3cc16f8403ae3d1daf2e1
+e997b0201512110e9c20b1fdfd40014830031047
 
 The first line of this file holds the git revision number of the last
 merge done from the gofrontend repository.