Fix PR33875 by distinguishing between DWO and clang modules

The DWO handling code can get confused by clang modules which also use
skeleton CUs to point to the object file with the full debug
info. This patch detects whether an object is a "real" DWO or a clang
module and prevents LLDB from interpreting clang modules as DWO. This
fixes the regression in TestWithModuleDebugging.

http://llvm.org/bugs/show_bug.cgi?id=33875

Differential Revision: https://reviews.llvm.org/D35740

llvm-svn: 308850