Work around a Visual C++ bug.

Work around a Visual C++ bug.

Using a static function as a template parameter gets a bogus compile-time
error with Visual Studio 2017, prior to version 15.8. Our current
minimum-version requirement is a particular update to VS2015, and we
assume all Visual Studio 2017 versions are usable. This patch makes the
code buildable with older versions of VS2017, and can be reverted after
we upgrade the minimum version sometime in the future.

Description of the Microsoft bug:
https://developercommunity.visualstudio.com/content/problem/25334/error-code-c2971-when-specifying-a-function-as-the.html

Differential Revision: https://reviews.llvm.org/D62202

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@361502 91177308-0d34-0410-b5e6-96231b3b80d8