CMake: -Wno-nested-anon-types for Clang.

In LLVM, -pedantic is not set unless LLVM_ENABLE_PEDANTIC is set.
However, Clang's CMakeLists.txt unilaterally adds -pedantic to the run
line, so we need to disable -Wnested-anon-types explicitly.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176393 91177308-0d34-0410-b5e6-96231b3b80d8