[checked-arithmetic builtins] Added builtins to enable users to perform checked-arithmetic in c.

This will enable users in security critical applications to perform
checked-arithmetic in a fast safe manner that is amenable to c.

Tests/an update to Language Extensions is included as well.

rdar://13421498.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184497 91177308-0d34-0410-b5e6-96231b3b80d8