Correct a standalone debug file path

For a file /bin/ls with a .gnu_debuglink entry of "ls.debug" the path
should be /usr/lib/debug/bin/ls.debug, not /usr/lib/debug/bin/ls.

ref: https://sourceware.org/gdb/onlinedocs/gdb/Separate-Debug-Files.html

llvm.org/pr17903
http://llvm-reviews.chandlerc.com/D2169

llvm-svn: 195681