When merging class definitions across modules in C++, merge together fields.

When merging class definitions across modules in C++, merge together fields.
This change doesn't go all the way to making fields redeclarable; instead, it
makes them 'mergeable', which means we can find the canonical declaration, but
not much else (and for a declaration that's not from a module, the canonical
declaration is always that declaration).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192092 91177308-0d34-0410-b5e6-96231b3b80d8