[cmake] Avoid rel+asserts warnings when passing -UNDEBUG

MSVC 2012 gives warning D9025, "overriding /D NDEBUG with -UNDEBUG".
Removing the original definition of NDEBUG silences this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178967 91177308-0d34-0410-b5e6-96231b3b80d8