[PATCH] Handle use of default member initializers before end of outermost class

[Reid Kleckner]

Hi rsmith,

Specifically, when we have this situation:
  struct A {
    template <typename T> struct B {
      int m1 = sizeof(A);
    };
    B<int> m2;
  };

We can't parse m1's initializer eagerly because we need A to be
complete.  Therefore we wait until the end of A's class scope to parse
it. However, we can trigger instantiation of B before the end of A,
which will attempt to instantiate the field decls eagerly, and it would
build a bad field decl instantiation that said it had an initializer but
actually lacked one.

Fixed by doing template instantiation when building a CXXDefaultInitExpr
and erorring out if the initializer is missing for any other reason.
Fixes PR19195.

This does *not* defer all field instantiation until later, as that
causes assertions in the test suite.  Fixing that might be the right
thing to do, but I wanted feedback on the approach so far first.

http://reviews.llvm.org/D5690

Files:
  include/clang/AST/ExprCXX.h
  include/clang/Basic/DiagnosticSemaKinds.td
  include/clang/Sema/Sema.h
  lib/Sema/SemaDeclCXX.cpp
  lib/Sema/SemaInit.cpp
  lib/Sema/SemaTemplateInstantiate.cpp
  lib/Sema/SemaTemplateInstantiateDecl.cpp
  test/SemaCXX/constant-expression-cxx11.cpp
  test/SemaCXX/implicit-exception-spec.cpp
  test/SemaCXX/member-init.cpp
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