When loading a PE/COFF target, the associated PDB file often wasn't found. The executable module contains a path for the associated PDB file, but people often debug from a different directory than the one their build system uses. (This is especially common in post-mortem and cross platform debugging.)
Suppose the COFF executable being debugged is ~/proj/foo.exe, but it was built elsewhere and refers to D:\remote\build\env\foobar.pdb, LLDB wouldn't find it.
With this change, if no file exists at the PDB path, LLDB will look in the executable directory for a PDB file that matches the name of the one it expected (e.g., ~/proj/foobar.pdb). If found, the PDB is subject to the same matching criteria (GUIDs and age) as would have been used had it been in the original location.
This same-directory-as-the-binary rule is commonly used by debuggers on Windows.