[cmake] Set CMP0091 to fix Windows builds after the cmake_minimum_req…

…uired bump

The build uses other mechanism to select the runtime.

Fixes #62719

Differential revision: https://reviews.llvm.org/D150688

cmake/Modules/CMakePolicy.cmake

@@ -1,5 +1,10 @@
 # CMake policy settings shared between LLVM projects
 
+# CMP0091: MSVC runtime library flags are selected by an abstraction.
+# New in CMake 3.15. https://cmake.org/cmake/help/latest/policy/CMP0091.html
+if(POLICY CMP0091)
+  cmake_policy(SET CMP0091 OLD)
+endif()
 # CMP0114: ExternalProject step targets fully adopt their steps.
 # New in CMake 3.19: https://cmake.org/cmake/help/latest/policy/CMP0114.html
 if(POLICY CMP0114)