cmake: disable -Wnon-virtual-dtor when it gives false positives

clang has only been smart enough not to trigger -Wnon-virtual-dtor
warnings on final classes since r208449 (in clang 3.5). Building
with older versions is extremely noisy, so disable the warning
on those compilers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216327 91177308-0d34-0410-b5e6-96231b3b80d8

cmake/modules/HandleLLVMOptions.cmake

@@ -288,8 +288,20 @@ elseif( LLVM_COMPILER_IS_GCC_COMPATIBLE )
     add_flag_if_supported("-Wcovered-switch-default" COVERED_SWITCH_DEFAULT_FLAG)
     append_if(USE_NO_UNINITIALIZED "-Wno-uninitialized" CMAKE_CXX_FLAGS)
     append_if(USE_NO_MAYBE_UNINITIALIZED "-Wno-maybe-uninitialized" CMAKE_CXX_FLAGS)
-    check_cxx_compiler_flag("-Werror -Wnon-virtual-dtor" CXX_SUPPORTS_NON_VIRTUAL_DTOR_FLAG)
-    append_if(CXX_SUPPORTS_NON_VIRTUAL_DTOR_FLAG "-Wnon-virtual-dtor" CMAKE_CXX_FLAGS)
+
+    # Check if -Wnon-virtual-dtor warns even though the class is marked final.
+    # If it does, don't add it. So it won't be added on clang 3.4 and older.
+    # This also catches cases when -Wnon-virtual-dtor isn't supported by
+    # the compiler at all.
+    set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
+    set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -std=c++11 -Werror=non-virtual-dtor")
+    CHECK_CXX_SOURCE_COMPILES("class base {public: virtual void anchor();protected: ~base();};
+                               class derived final : public base { public: ~derived();};
+                               int main() { return 0; }"
+                              CXX_WONT_WARN_ON_FINAL_NONVIRTUALDTOR)
+    set(CMAKE_REQUIRED_FLAGS ${OLD_CMAKE_REQUIRED_FLAGS})
+    append_if(CXX_WONT_WARN_ON_FINAL_NONVIRTUALDTOR
+              "-Wnon-virtual-dtor" CMAKE_CXX_FLAGS)
 
     # Check if -Wcomment is OK with an // comment ending with '\' if the next
     # line is also a // comment.