cmd/compile: fix logopt log directory naming for windows
Allow Windows absolute paths, also fixed URI decoding on Windows.
Added a test, reorganized to make the test cleaner.
Also put some doc comments on exported functions that did not have them.
Fixes #41614.
Change-Id: I2871be0e5183fbd53ffb309896d6fe56c15a7727
Reviewed-on: https://go-review.googlesource.com/c/go/+/257677
Trust: David Chase <drchase@google.com>
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Go Bot <gobot@golang.org> Reviewed-by: Rebecca Stambler <rstambler@golang.org>