cmake: Only add -pedantic if LLVM didn't add it. Don't unconditionally add
-Wall -W, since it's already provided by LLVM's cmake config, and that
overrides fixes (such as -Wno-uninitialized) which LLVM's cmake setup may have
provided.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@180018 91177308-0d34-0410-b5e6-96231b3b80d8