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.
git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@161308 91177308-0d34-0410-b5e6-96231b3b80d8
