Modules: Code generation of enum constants for merged enum definitions

Found in a bootstrap of LLVM with implicit modules, resulting in a
deadlock of some Orc unit tests with libstdc++ 8.1. An enum was used as
part of the implementation of std::recursive_mutex and this bug resulted
in the constant initialization of zero instead of the desired non-zero
value. => Badness.

Richard Smith tells me neither of these fields are necessarily canonical
& so using declaresSamEntity is the right solution here (rather than
changing both of these Fields to be canonical by construction/from their
source)

llvm-svn: 361428