compiler: make comparison operator() methods const

This is required for new versions of libgo in C++17 mode.

Fixes GCC PR 83102.

Change-Id: Iba7a9010b9fb748e0b93f54e25941d776015388f
Reviewed-on: https://go-review.googlesource.com/79396
Reviewed-by: Ian Lance Taylor <iant@golang.org>
3 files changed