[LLDB] Improve PDB discovery

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.

Differential Review: https://reviews.llvm.org/D84815