Module Debugging: Use the clang module signature as the module's dwo_id

when building a module. Clang already records the module signature when
building a skeleton CU to reference a clang module.

Matching the id in the skeleton with the one in the module allows a DWARF
consumer to verify that they found the correct version of the module
without them needing to know about the clang module format.

llvm-svn: 248345