compiler: move Selector_expression up in file

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