[cmake] Add -fms-compatibility-version=19 when clang-cl gives errors about char16_t

What we are really trying to do here is to figure out if we are using
the 2015 STL. Unfortunately, so far as I know the MSVC STL does not
define a version macro that we can check directly. Instead I wrote a
check to see if char16_t works.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264881 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed