LLVM_ENABLE_ASSERTIONS instead of LIBCXX_ENABLE_ASSERTIONS when
figuring out what _DEBUG/NDEBUG defines to set. It also tries to test
the non-existent variable 'uppercase_CMAKE_BUILD_TYPE', which the top
level LLVM CMakeLists.txt sets up, but which the top level libc++
CMakeLists.txt currently does not. Changing the variable name tested
and creating the uppercase release name variable allows libc++ to
honor the LIBCXX_ENABLE_ASSERTIONS option correctly.

llvm-svn: 161308