ObjectiveC. Define a new cc1 flag 
-fobjc-subscripting-legacy-runtime which is off 
by default and on only when using ObjectiveC
legacy runtime. Use this flag to allow
array and dictionary subscripting and disallow
objectiveC pointer arithmatic in ObjectiveC
legacy runtime. // rdar://15363492


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193889 91177308-0d34-0410-b5e6-96231b3b80d8