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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@248345 91177308-0d34-0410-b5e6-96231b3b80d8