cmake: Don't do anything for LLVM_ENABLE_ASSERTIONS=OFF

By default, CMake will set NDEBUG in Rel* builds and leave it off in
debug builds, so we shouldn't need to do anything ourselves.

Before this change, it was possible to a Debug build without assertions
(aka Debug-Asserts in the autoconf system) by configuring with
-DLLVM_ENABLE_ASSERTIONS=OFF, but this configuration isn't very useful.
You can still get the same effect by explicitly adding -DNDEBUG to
CFLAGS.

Differential Revision: http://reviews.llvm.org/D4257

Patch by Janusz Sobczak!