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.

llvm-svn: 176393