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!
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211919
91177308-0d34-0410-b5e6-
96231b3b80d8