[clang] Silence some warnings when building clang with external llvm

If you build clang with `builtin_llvm=OFF` you might get flooded with so
many warnings that it's unsustainable.

This commit suggests to silence these kind of warnings. Note that there
are still some warnings left, but not to the point that the output is
completely flooding.

This is an upstreamed patch of what is done for nikpkgs:
https://github.com/NixOS/nixpkgs/blob/master/pkgs/applications/science/misc/root/default.nix#L200

Author: guitargeek

interpreter/llvm-project/clang/CMakeLists.txt

@@ -310,7 +310,7 @@ if (LLVM_COMPILER_IS_GCC_COMPATIBLE)
 
   # Enable -pedantic for Clang even if it's not enabled for LLVM.
   if (NOT LLVM_ENABLE_PEDANTIC)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wno-long-long")
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pedantic -Wno-long-long -Wno-shadow -Wno-maybe-uninitialized")
   endif ()
 
   check_cxx_compiler_flag("-Werror -Wnested-anon-types" CXX_SUPPORTS_NO_NESTED_ANON_TYPES_FLAG)