Merge pull request swiftlang#34449 from apple/build-Use-llvm-matching-value-for-CMake-policy-CMP0077

[build] Use llvm matching value for CMake policy CMP0077

Author: kastiglione

CMakeLists.txt

@@ -17,6 +17,11 @@ if(POLICY CMP0076)
   cmake_policy(SET CMP0076 NEW)
 endif()
 
+# Don't clobber existing variable values when evaluating `option()` declarations.
+if(POLICY CMP0077)
+  cmake_policy(SET CMP0077 NEW)
+endif()
+
 # Add path for custom CMake modules.
 list(APPEND CMAKE_MODULE_PATH
     "${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules")