This is similar to r245139, but that only addressed dllexported classes. It was still possible to run into the same problem with dllexported members in an otherwise normal class (see bug). This uses the same strategy to fix: delay defining the method until the whole class has been parsed.
(The easiest way to see the ordering problem is in Parser::ParseCXXMemberSpecification(): it calls ParseLexedMemberInitializers() *after* ActOnFinishCXXMemberDecls(), which was trying to define the dllexport method. Now we delay it to ActOnFinishCXXNonNestedClass() which is called after both of those.)