[CMake] Enable a bunch of Xcode build settings that correspond to warnings that are for the most part enabled by default either by Clang or -Wall.

I personally build with these settings enabled all the time, and it
is clearer to see the actual warning flags (e.g., -Wuninitialized)
get passed by Xcode rather than seeing -Wno-uninitialized followed
by -Wall (the latter canceling out the former) and figuring out
what is going on.

Xcode will ignore build settings it doesn't understand, so this will
work on possibly older versions of Xcode that don't support all
of these settings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203760 91177308-0d34-0410-b5e6-96231b3b80d8