Removing SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG

Adding CXX_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG
       C_SUPPORTS_COVERED_SWITCH_DEFAULT_FLAG

This is to handle the wackiness on a Mac host where cmake detects:

CMAKE_CXX_COMPILER == "/usr/bin/c++"
CMAKE_C_COMPILER == "/usr/bin/gcc"

llvm-svn: 168577