Properly set the scope of non-fields declared within a struct, union,

or enum to be outside that struct, union, or enum. Fixes several
regressions: 

  <rdar://problem/6487662>
  <rdar://problem/6487669>
  <rdar://problem/6487684>
  <rdar://problem/6487702>
  PR clang/3305
  PR clang/3312

There is still some work to do in Objective-C++, but this requires
that each of the Objective-C entities (interfaces, implementations,
etc.) to be introduced into the context stack with
PushDeclContext/PopDeclContext. This will be a separate fix, later.

llvm-svn: 62091