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.