[cmake] Fix error message on simulated VS version check

For clang-cl self hosts in VS2015 environment this was reporting: "Host
Clang must have at least -fms-compatibility-version=19.00.24213.1, your
version is 9.0.0".

This check fires as CMake detects the simulated environment as _MSC_VER
1900, which is truncated. This makes it less than the required
19.00.24213.1.

Differential revision: https://reviews.llvm.org/D61188

llvm-svn: 359556