Module Debugging: Use a nonzero DWO id for precompiled headers.

PCH files don't have a module signature and LLVM uses a nonzero DWO id as
an indicator for skeleton / module CUs. This change pins the DWO id for PCH
files to a known constant value.
The correct long-term solution here is to implement a module signature
that is an actual dterministic hash (at the moment module signatures are
just random nonzero numbers) and then enable this for PCH files as well.

<rdar://problem/24290667>

llvm-svn: 258507