[modules] PR28812: Modules can return duplicate field decls.

If two modules contain duplicate class definitions the lookup result can contain
more than 2 elements. Sift the lookup results until we find a field decl.

It is not necessary to do ODR checks in place as they done elsewhere.

This should fix issues when compiling with libstdc++ 5.2 and 6.2.

Patch developed in collaboration with Richard Smith!

llvm-svn: 285184