Merging r369043:

------------------------------------------------------------------------
r369043 | rnk | 2019-08-15 21:45:28 +0200 (Thu, 15 Aug 2019) | 15 lines

[Sema] Implement DR2386 for C++17 structured binding

Allow implementations to provide complete definitions of
std::tuple_size<T>, but to omit the 'value' member to signal that T is
not tuple-like. The Microsoft standard library implements
std::tuple_size<const T> this way.

If the value member exists, clang still validates that it is an ICE, but
if it does not, then the type is considered to not be tuple-like.

Fixes PR33236

Reviewers: rsmith

Differential Revision: https://reviews.llvm.org/D66040
------------------------------------------------------------------------

llvm-svn: 369361