[CMake] Infer -Werror=switch via asserts being enabled (#11098)

LLVM-style projects like Swift make heavy use of switch statements and
therefore -Werror=switch is very useful during feature development. That
being said, if asserts are disabled, then you're probably not actively
hacking on the project and -Werror (or warnings in general) aren't
helpful.

Author: davezarzycki

cmake/modules/SwiftSharedCMakeConfig.cmake

@@ -256,6 +256,11 @@ endmacro()
 # Common cmake project config for additional warnings.
 #
 macro(swift_common_cxx_warnings)
+  # Make unhandled switch cases be an error in assert builds
+  if(DEFINED LLVM_ENABLE_ASSERTIONS)
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Werror=switch")
+  endif()
+
   check_cxx_compiler_flag("-Werror -Wdocumentation" CXX_SUPPORTS_DOCUMENTATION_FLAG)
   append_if(CXX_SUPPORTS_DOCUMENTATION_FLAG "-Wdocumentation" CMAKE_CXX_FLAGS)