Set CMP0157 to OLD only for Windows hosts (#5195)

- **Explanation**: There is no early swift-driver build for the Windows toolchain. As a result, swift-collections fails to build properly when CMP0157 is set to NEW due to object files not being generated. This sets CMP0157 to OLD when targetting Android until the early swift-driver is available on Windows. - **Scope**: The changes are limited to the Android build on Windows. - **Issues**: - swiftlang/swift-foundation#1222 - **Original PRs**: #5180 - **Risk**: Very low. - **Testing**: Fixes the Swift CI 6.1 build. - **Reviewers**: @finagolfin

Author: Steelskin

CMakeLists.txt

@@ -22,8 +22,15 @@ if(POLICY CMP0156)
 endif()
 
 if(POLICY CMP0157)
-# New Swift build model: improved incremental build performance and LSP support
- cmake_policy(SET CMP0157 NEW)
+ if(CMAKE_HOST_SYSTEM_NAMESTREQUAL Windows ANDCMAKE_SYSTEM_NAMESTREQUAL Android)
+# CMP0157 causes swift-collections to fail to compile when targetting
+# Android on Windows due to swift-driver not being present during the
+# toolchain build. Disable it for now.
+ cmake_policy(SET CMP0157 OLD)
+ else()
+# New Swift build model: improved incremental build performance and LSP support
+ cmake_policy(SET CMP0157 NEW)
+ endif()
 endif()
 
 if (NOTDEFINED CMAKE_C_COMPILER)