Go's DWARF usually has absolute paths, in which case DW_AT_comp_dir
doesn't matter. But the -trimpath flag produces relative paths, and
then the spec says that they are relative to _comp_dir.

There's no way to know what the "right" value of _comp_dir is without
more user input, but we can at least leave the paths alone rather than
making them absolute.

After this change, Delve can find sources to a program built with
-gcflags=-trimpath=$(pwd) as long as it's run in the right directory.

Change-Id: I8bc7bed098e352d2c06800bfbbe14e8392e1bbed
Reviewed-on: https://go-review.googlesource.com/78415
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>