Symbols::LocateExecutableSymbolFile() shouldn't try to look for files in /usr/lib/debug on Windows

Summary:
/usr/lib/debug doesn't exist on Windows so there's no point even
attempting to look for symbol files in there.

Reviewers: zturner, clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D13636

llvm-svn: 250175