Don't use add_llvm_definitions for adding warning flags. It makes it too hard

Don't use add_llvm_definitions for adding warning flags.  It makes it too hard
to have them appear in the right order.  Instead append all warnings explicitly
to the language flags.  This was already the case for many warnings.  Fixes the
issue of -Wno-maybe-uninitialized not being effective because -Wall was being
placed after it rather than before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177866 91177308-0d34-0410-b5e6-96231b3b80d8