[CMake][llvm] avoid changing global flags (may be used outside of llvm)

Changing global flags can break builds of projects that include/build
llvm as a sub-project, as the effect is global. Ideally we would
disable this warning at the directory level instead, but the obvious
way (disabling warning D9025) isn't supported. At least we can limit
the effect to only MSVC.

Patch by Jim Radford.

Differential Revision: https://reviews.llvm.org/D100900