[llvm-dwp] Join dwo paths correctly when DWOPath is absolute (12.0)

[nagisa]

When the DWOPath is absolute, we want to use DWOPath as is, without prepending any other
components to the path. The sys::path::append does not join, but rather unconditionally appends
the paths, so something like sys::path::append("/tmp", "/tmp/banana") will result in
/tmp/tmp/banana rather than the desired /tmp/banana.

This then causes llvm-dwp to fail in a following situation:

$ clang -gsplit-dwarf /tmp/banana/test.c -c -o /tmp/outdir/foo.o
$ clang outdir/foo.o -o outdir/hm
$ llvm-dwarfdump outdir/hm | grep -C2 foo.dwo
                  DW_AT_comp_dir    ("/tmp")
                  DW_AT_GNU_pubnames  (true)
                  DW_AT_GNU_dwo_name    ("/tmp/outdir/foo.dwo")
                                DW_AT_GNU_dwo_id    (0xde4d396f3bf0e257)
                  DW_AT_low_pc  (0x0000000000401100)
$ strace -o trace llvm-dwp -e outdir/hm -o outdir/hm.dwp
error: No such file or directory
$ cat trace | grep foo.dwo
openat(AT_FDCWD, "/tmp/tmp/outdir/foo.dwo", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)

Reviewed By: dblaikie

Differential Revision: https://reviews.llvm.org/D96678

Backport of: 6ffcb29

[nikic]

Did you request a backport to the release/12.x branch for this change?

[nagisa]

I did not.

[nikic]

Okay, I'll file one then. Is there any context on what this fixes on the rustc side? I couldn't find a relevant issue.

[nagisa]

Ah, rust-lang/rust#82102 is the PR that this is necessary for.

[nagisa]

But also see https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/llvm-dwp.20is.20not.20recommended/near/226567575 which potentially means we'll look to migrate away from this tool in some way or another.

[nikic]

Thanks! I've opened https://bugs.llvm.org/show_bug.cgi?id=49229 for the backport.