[Modules] Handle ObjC/C ODR-like semantics for EnumConstantDecl

Support for ObjC/C ODR-like semantics with structural equivalence
checking was added back in r306918. There enums are handled and also
checked for structural equivalence. However, at use time of
EnumConstantDecl, support was missing for preventing ambiguous
name lookup.

Add the missing bits for properly merging EnumConstantDecl.

rdar://problem/38374569

llvm-svn: 331232