[DataFormatters] Add formatter for C++17 std::variant

rdar://problem/43691454

Patch by Shafik Yaghmour.

Differential Revision: https://reviews.llvm.org/D51520

git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@342421 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed