Handle use of default member initializers before end of outermost class

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 deferring instantiation of default member initializers until
they are needed during constructor analysis. This addresses a long
standing FIXME in the code.

Fixes PR19195.

Reviewed By: rsmith

Differential Revision: http://reviews.llvm.org/D5690

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222192 91177308-0d34-0410-b5e6-96231b3b80d8
Make DiagnosticsEngine::takeClient return std::unique_ptr<>

Summary:
Make DiagnosticsEngine::takeClient return std::unique_ptr<>. Updated
callers to store conditional ownership using a pair of pointer and unique_ptr
instead of a pointer + bool. Updated code that temporarily registers clients to
use the non-owning registration (+ removed extra calls to takeClient).

Reviewers: dblaikie

Reviewed By: dblaikie

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6294

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222193 91177308-0d34-0410-b5e6-96231b3b80d8