[MS] Raise the default value of _MSC_VER to 1911, which is VS 2017

Summary:
This raises our default past 1900, which controls whether char16_t is a
builtin type or not.

Implements PR34243

Reviewers: hans

Subscribers: STL_MSFT, rsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D38646

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315107 91177308-0d34-0410-b5e6-96231b3b80d8