[test] Turn off warnings for test/Analysis/identical-expressions.cpp.

Also, make it slightly clearer what's being tested by only differentiating integer
literals based on their suffix, rather than using a very large constant.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@216133 91177308-0d34-0410-b5e6-96231b3b80d8