[DebugInfo] Don't use realpath when looking up debug binary locations.

Summary:
Using realpath makes assumptions about build systems that do not always hold true. The debug binary referred to from the .gnu_debuglink should exist in the same directory (or in a .debug directory, etc.), but the files may only exist as symlinks to a differently named files elsewhere, and using realpath causes that lookup to fail.

This was added in r189250, and this is basically a revert + regression test case.

Reviewers: dblaikie, samsonov, jhenderson

Reviewed By: dblaikie

Subscribers: llvm-commits, hiraditya

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D57609

llvm-svn: 352916