Add an `__is_inplace_index` metafunction.

Summary: This is used to constrain `variant`'s converting constructor correctly.

Reviewers: EricWF, mclow.lists

Reviewed By: EricWF, mclow.lists

Differential Revision: https://reviews.llvm.org/D34111

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@305370 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed