Enable c++1z experimental builds.

Patch by David Abdurachmanov!

llvm-svn: 305123
diff --git a/llvm/cmake/modules/HandleLLVMOptions.cmake b/llvm/cmake/modules/HandleLLVMOptions.cmake
index d159331..c3325db 100644
--- a/llvm/cmake/modules/HandleLLVMOptions.cmake
+++ b/llvm/cmake/modules/HandleLLVMOptions.cmake
@@ -385,6 +385,9 @@
   if (LLVM_ENABLE_CXX1Y)
     check_cxx_compiler_flag("-std=c++1y" CXX_SUPPORTS_CXX1Y)
     append_if(CXX_SUPPORTS_CXX1Y "-std=c++1y" CMAKE_CXX_FLAGS)
+  elseif(LLVM_ENABLE_CXX1Z)
+    check_cxx_compiler_flag("-std=c++1z" CXX_SUPPORTS_CXX1Z)
+    append_if(CXX_SUPPORTS_CXX1Z "-std=c++1z" CMAKE_CXX_FLAGS)
   else()
     check_cxx_compiler_flag("-std=c++11" CXX_SUPPORTS_CXX11)
     if (CXX_SUPPORTS_CXX11)