[llvm-symbolizer] Use correct path when resolving .gnu_debuglink in .debug

Summary:
The symbolizer was checking for .debug as a subdirectory of the
binary file itself, not of the directory containing the binary. This led to
a failure to find split debug info when it was contained in a .debug directory.

Reviewers: rnk, glider, zturner

Subscribers: llvm-commits, aprantl

Differential Revision: https://reviews.llvm.org/D44025

llvm-svn: 326630