[LLVM-C] Add LLVM{IsConstantString,GetAsString,GetElementAsConstant}.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214676 91177308-0d34-0410-b5e6-96231b3b80d8
[OCaml] Add Llvm.{string_of_const,const_element}.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214677 91177308-0d34-0410-b5e6-96231b3b80d8