[DebugInfo] Don't use realpath when looking up debug binary locations.
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
Differential Revision: https://reviews.llvm.org/D57609