Unify the code for defining tags in C and C++, so that we always

introduce a Scope for the body of a tag. This reduces the number of
semantic differences between C and C++ structs and unions, and will
help with other features (e.g., anonymous unions) in C. Some important
points:

  - Fields are now in the "member" namespace (IDNS_Member), to keep
    them separate from tags and ordinary names in C. See the new test
    in Sema/member-reference.c for an example of why this matters. In
    C++, ordinary and member name lookup will find members in both the
    ordinary and member namespace, so the difference between
    IDNS_Member and IDNS_Ordinary is erased by Sema::LookupDecl (but
    only in C++!). 
  - We always introduce a Scope and push a DeclContext when we're
    defining a tag, in both C and C++. Previously, we had different
    actions and different Scope/CurContext behavior for enums, C
    structs/unions, and C++ structs/unions/classes. Now, it's one pair
    of actions. (Yay!)

There's still some fuzziness in the handling of struct/union/enum
definitions within other struct/union/enum definitions in C. We'll
need to do some more cleanup to eliminate some reliance on CurContext
before we can solve this issue for real. What we want is for something
like this:

  struct X {
    struct T { int x; } t;
  };

to introduce T into translation unit scope (placing it at the
appropriate point in the IdentifierResolver chain, too), but it should
still have struct X as its lexical declaration
context. PushOnScopeChains isn't smart enough to do that yet, though,
so there's a FIXME test in nested-redef.c

llvm-svn: 61940