Added a workaround for a `-fdelayed-template-parsing` bug.

Summary:
There seems to be an additional bug in `-fdelayed-template-parsing`
similar to
http://llvm.org/viewvc/llvm-project?view=revision&revision=236063.

This is a workaround for it for <variant> to compile with `clang-cl` on Windows.

Reviewers: EricWF

Differential Revision: https://reviews.llvm.org/D28734

llvm-svn: 292097