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

[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