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

[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.

llvm-svn: 264881